---
title: "Can one inductor feed three rails?"
topic: "SIMO converters"
answer: "Yes, if the rails take turns. A single-inductor multiple-output converter, or SIMO, runs this watch’s three rails for 8.6 h, against 8.8 h from three bucks and 4.3 h from three LDOs. It takes 61% of the bucks’ board area."
url: "https://nearfieldnotes.com/notes/simo/"
site: "Near Field Notes"
author: "Michael Walker"
note: 22
family: "Power"
published: "2026-11-06"
revised: "2026-11-06"
revision: 1
video: "https://nearfieldnotes.com/videos/simo/"
---

Note 22 · [Power](https://nearfieldnotes.com/families/power/) · Rev 1 · [Watch the video · 0:44](https://nearfieldnotes.com/videos/simo/)

SIMO converters

# Can one inductor feed three rails?

**Short answer:** Yes, if the rails take turns. A single-inductor multiple-output converter, or SIMO, runs this watch’s three rails for 8.6 h, against 8.8 h from three bucks and 4.3 h from three LDOs. It takes 61% of the bucks’ board area.

The figure is a model of the MAX77654 at work. Its one 1.5 µH inductor fills from the cell, then empties into whichever rail asks, one packet at a time.

**Note 22’s opening figure: One inductor, three rails.** 1.1 and 1.8 V in buck mode, 3.3 V in buck-boost

![Note 22’s opening figure: One inductor, three rails](https://nearfieldnotes.com/notes/simo/fig-opening.png)

*What the figure shows:* The SIMO's power stage: a 3.7 V cell, switches M1 and M2, one 1.5 µH inductor, switch M4, and switches M3_1, M3_0 and M3_2 to the 1.1, 1.8 and 3.3 V rails. Under it, the inductor's current over 34 µs: 15 packets, each a hump of current to one rail, the 3.3 V ones to 1 A and the others to 0.5 A. Under that, each rail's voltage against its set point, rising about 20 mV at each of its packets and sagging between them. At 12 µs the 1.8 V load steps from 20 to 60 mA and its packets come more often.

Each colored hump is one packet: the inductor fills, then empties into one rail. Each rail climbs about 20 mV with its packet, then sags until it asks again. At the dotted line the 1.8 V load steps from 20 to 60 mA.

**Every packet**

| Packet | Rail | Mode | Starts | Peak current | Ends |
|---|---|---|---|---|---|
| 1 | 1.1 V | buck mode | 2.50 µs | 0.5 A | 3.42 µs |
| 2 | 1.8 V | buck mode | 5.90 µs | 0.5 A | 6.71 µs |
| 3 | 3.3 V | buck-boost | 6.71 µs | 1.0 A | 7.57 µs |
| 4 | 1.1 V | buck mode | 8.19 µs | 0.5 A | 9.11 µs |
| 5 | 1.8 V | buck mode | 13.35 µs | 0.5 A | 14.16 µs |
| 6 | 1.1 V | buck mode | 14.16 µs | 0.5 A | 15.08 µs |
| 7 | 1.8 V | buck mode | 16.73 µs | 0.5 A | 17.54 µs |
| 8 | 1.1 V | buck mode | 19.56 µs | 0.5 A | 20.48 µs |
| 9 | 1.8 V | buck mode | 20.48 µs | 0.5 A | 21.29 µs |
| 10 | 1.8 V | buck mode | 23.50 µs | 0.5 A | 24.31 µs |
| 11 | 1.1 V | buck mode | 25.25 µs | 0.5 A | 26.17 µs |
| 12 | 1.8 V | buck mode | 26.88 µs | 0.5 A | 27.69 µs |
| 13 | 3.3 V | buck-boost | 27.69 µs | 1.0 A | 28.55 µs |
| 14 | 1.8 V | buck mode | 30.27 µs | 0.5 A | 31.08 µs |
| 15 | 1.1 V | buck mode | 31.08 µs | 0.5 A | 32.00 µs |

## A crowded watch board

*Three rails from one cell*

The board is a round watch board, 22 mm across and packed the way a watch’s is. An MCU, a chip antenna, the display’s connector, an IMU, a heart-rate sensor, a microphone and a fuel gauge leave 33 mm² for the parts that make the rails.

A 300 mAh lithium-ion cell at 3.7 V feeds three rails. The MCU’s core takes 40 mA at 1.1 V. Its I/O takes 20 mA at 1.8 V. The sensors take 10 mA at 3.3 V. That’s 113 mW in all.

**Fig. 22: The watch board.**

![Fig. 22: The watch board](https://nearfieldnotes.com/notes/simo/fig-22.png)

*What the figure shows:* A round 22 mm watch board rendered from above, packed with parts: a chip antenna at the top, the display connector, the MCU, an IMU, a microphone, and a dashed box at the left of center, the 33 mm² left for power, here holding one MAX77654 and its inductor. The battery's two tabs come in from the left.

The SIMO version, rendered from its KiCad layout. The dashed box is the 33 mm² left for power. The cell’s tabs come in from the left.

**The numbers**

| Part | Where, mm from top left |
|---|---|
| Antenna | 11.2, 2.1 |
| Display connector | 6.4, 4.9 |
| MCU | 15.7, 8.4 |
| IMU | 15.5, 14.4 |
| Microphone | 7.2, 16.1 |
| Room for power | 4.30 to 10.95, 7.39 to 12.41. 33.4 mm² |

There are three usual ways to make the rails. An [LDO](https://nearfieldnotes.com/glossary/#g-ldo) is the smallest, a 0.64 mm chip with a capacitor each side. A [buck](https://nearfieldnotes.com/glossary/#g-buck) needs an inductor of its own. A [SIMO](https://nearfieldnotes.com/glossary/#g-simo) shares one inductor among all three rails.

At one scale, two bucks fill the room and the third has nowhere to go. The SIMO and its five capacitors fit.

**Fig. 22b: The power section, three ways.**

![Fig. 22b: The power section, three ways](https://nearfieldnotes.com/notes/simo/fig-22b.png)

*What the figure shows:* The power section of the watch board three ways, at one scale, with the dashed 33 mm² room for power in each. Three LDOs take 10.8 mm². Three bucks take 38.7 mm²: two fit, and the third, outlined, lands on the fuel gauge, an LED and the microphone below. One SIMO takes 23.5 mm².

The same 8 mm of board each time. Areas are the parts’ courtyards. The outlined third buck lands on the fuel gauge, an LED and the microphone.

**The numbers**

| Way | Parts | Courtyards | Fits the room |
|---|---|---|---|
| Three LDOs | 3 TPS7A02, 6 capacitors | 10.8 mm² | Yes |
| Three bucks | 1 TPS62841, 2 TPS62840, 3 inductors, 6 capacitors, 2 resistors | 38.7 mm² | No: the third lands on 6 parts |
| One SIMO | 1 MAX77654, 1 inductor, 5 capacitors | 23.5 mm² | Yes |

## Battery life and area

*What each way costs the watch*

An LDO passes its load’s current straight through from the cell. With three of them, battery life is the cell’s charge over the total current:

`t = Q/I`

where t: battery life (h); Q: the cell’s charge (mAh); I: total current from the cell (mA).

`= (300 mAh)/(70 mA) = 4.3 h`

A buck or a SIMO converts power instead, and loses a little on the way. Its battery life is the cell’s energy over the power it draws:

`t = (Q · V_cell)/P_in`

where t: battery life (h); Q: the cell’s charge (mAh); V_cell: the cell’s voltage (V); P_in: power drawn from the cell (mW).

`= (300 mAh × 3.7 V)/(129.2 mW) = 8.6 h`

Here *P*_in is each rail’s power over its converter’s efficiency at that load, read off the datasheets’ curves, plus the SIMO’s own bias current.

The bucks and the SIMO both double the LDOs’ battery life. The bucks run 12 minutes longer.

The 3.3 V rail alone accounts for more than the whole gap. There the MAX77654’s curve gives 85%, against 93% for a TPS62840. On the other two rails the SIMO does a little better.

The SIMO trades those minutes for 15.2 mm² of board, room the bucks never had.

**Fig. 22c: Battery life and board area.**

![Fig. 22c: Battery life and board area](https://nearfieldnotes.com/notes/simo/fig-22c.png)

*What the figure shows:* Bars of battery life and board area for the watch's three rails. Three LDOs: 4.3 h in 10.8 mm². Three bucks: 8.8 h in 38.7 mm², more than the 33 mm² room. One SIMO: 8.6 h in 23.5 mm².

The three rails’ 113 mW from a 300 mAh cell at 3.7 V. The dashed line is the room left for power.

**The numbers**

| Way | Battery power | Battery life | Board area |
|---|---|---|---|
| Three LDOs | 259.0 mW | 4.29 h | 10.8 mm² |
| Three bucks | 126.4 mW | 8.79 h | 38.7 mm² |
| One SIMO | 129.2 mW | 8.59 h | 23.5 mm² |

## Heat, and no room

*Why LDOs waste and bucks don’t fit*

An LDO drops the difference between the cell and its rail across itself, and turns it into heat:

`P_heat = (V_cell − V_out) · I_out`

where P_heat: an LDO’s heat (mW); V_cell: the cell’s voltage (V); V_out: the rail’s voltage (V); I_out: the rail’s load current (mA).

`= (3.7 V − 1.1 V) × 40 mA = 104 mW`

The 1.1 V rail is the costly one. It delivers 44 mW and burns 104 mW doing it. The 3.3 V LDO sits close under the cell and loses only 4 mW.

**Fig. 22d: Where the heat comes from.**

![Fig. 22d: Where the heat comes from](https://nearfieldnotes.com/notes/simo/fig-22d.png)

*What the figure shows:* Heat from making the three rails, by rail. Three LDOs: 146 mW, 104 of it in the 1.1 V rail. Three bucks: 13.4 mW. One SIMO: 16.2 mW.

Heat made by each way, split by rail. The SIMO’s total includes 0.14 mW of bias current.

**The numbers**

| Way | 1.1 V rail | 1.8 V rail | 3.3 V rail | Quiescent | All |
|---|---|---|---|---|---|
| Three LDOs | 104.0 mW | 38.0 mW | 4.0 mW | 0.28 µW | 146.0 mW |
| Three bucks | 6.9 mW | 4.0 mW | 2.5 mW | 0.67 µW | 13.4 mW |
| One SIMO | 6.7 mW | 3.6 mW | 5.8 mW | 0.14 mW | 16.2 mW |

Low-dropout regulator

### Tiny and simple, and it burns the difference

**Field marks.** A chip under a millimeter square with a capacitor each side, and no inductor. The TPS7A02 is 0.64 mm square.

**Look-alike.** A low quiescent current read as low loss. The TPS7A02’s 25 nA costs 0.09 µW here. The 2.6 V it drops at 40 mA costs 104 mW.

**Range.** Up to 200 mA, from 1.5 to 6 V in. Its efficiency is its output voltage over its input, so 30% for 1.1 V from 3.7 V.

**Habitat.** Wearables, meters, alarms and medical monitors, in TI’s list. Rails close under the cell, and loads of microamps.

Three bucks lose only 13.4 mW. But each needs a 2.0 by 1.6 mm inductor, and their parts’ courtyards add up to 38.7 mm², more than the 33 mm² room. In Fig. 22b the third buck lands on the fuel gauge, an LED and the microphone.

## How a packet works

*Fill, then empty*

A SIMO has one inductor and a switch to each rail. Each time a rail asks, it gets one packet. The inductor fills to a set peak current, then empties into that rail until its current is back to zero. Then it rests until a rail asks again.

The MAX77654 can fill two ways. In buck-boost it fills to ground through M1 and M4, and the rail gets only the emptying half. In buck mode it fills straight into the rail through M1 and that rail’s output switch, M3_x, as a plain buck does, and the rail gets both halves.

Buck mode needs the cell at least 0.7 V above the rail. That’s why the datasheet’s typical circuit runs 1.1 and 1.8 V in buck mode and 3.3 V in buck-boost, as the opening figure does. Buck-boost also keeps the 3.3 V rail up as the cell runs down below 3.3 V.

**Fig. 22e: Where the current flows.**

![Fig. 22e: Where the current flows](https://nearfieldnotes.com/notes/simo/fig-22e.png)

*What the figure shows:* Three copies of the SIMO's power stage with one output, the 1.1 V rail, each with one current path lit. Filling in buck-boost: M1 and M4 closed, from the cell through the inductor to ground. Filling in buck mode: M1 and M3_1 closed, from the cell through the inductor into the rail. Emptying, in either mode: M2 and M3_1 closed, from ground through the inductor into the rail.

The 1.1 V rail’s path only, through its output switch M3₁. The lit path carries the current. M4 and M3₁ are on the inductor’s rail side.

**The numbers**

| Step | Closed | Current flows | The rail gets |
|---|---|---|---|
| Fill, buck-boost | M1, M4 | Cell, inductor, ground | Nothing |
| Fill, buck mode | M1, M3₁ | Cell, inductor, rail | The rising current |
| Empty, either mode | M2, M3₁ | Ground, inductor, rail | The falling current |
| Rest | None | Nothing | Nothing |

A buck-boost packet hands its rail the charge the inductor gives up as it empties:

`q_bb = (L · I_pk^2)/(2 · V_out)`

where q_bb: charge a buck-boost packet gives its rail (µC); L: inductance (µH); I_pk: peak current (A); V_out: the rail’s voltage (V).

`= (1.5 µH × (0.5 A)^2)/(2 × 1.1 V) = 0.17 µC`

In buck mode the fill adds its own share, more the closer the rail sits to the cell:

`q_buck = q_bb · V_cell/(V_cell − V_out)`

where q_buck: charge a buck-mode packet gives its rail (µC); q_bb: charge a buck-boost packet gives its rail (µC); V_cell: the cell’s voltage (V); V_out: the rail’s voltage (V).

`= 0.17 µC × (3.7 V)/(3.7 V − 1.1 V) = 0.24 µC`

The sim gives 0.15 µC and 0.23 µC. Its switch and inductor resistance take a little off each packet.

**Fig. 22f: One packet, both ways.**

![Fig. 22f: One packet, both ways](https://nearfieldnotes.com/notes/simo/fig-22f.png)

*What the figure shows:* The inductor's current through one packet to the 1.1 V rail, rising to 0.5 A and back to zero. In buck-boost only the emptying half reaches the rail, 0.15 µC, over 832 ns. In buck mode the filling half reaches it too, 0.23 µC, over 920 ns.

One 1.1 V packet at a 0.5 A peak. The line is the inductor’s current. The filled part is the current that reaches the rail, and the number is its charge.

**The numbers**

| Mode | Peak at | Ends at | Charge to 1.1 V | Lifts 10 µF by | Energy in | Energy out |
|---|---|---|---|---|---|---|
| buck-boost | 208 ns | 832 ns | 0.152 µC | 15.2 mV | 196 nJ | 168 nJ |
| buck mode | 299 ns | 920 ns | 0.227 µC | 22.7 mV | 281 nJ | 252 nJ |

SIMO converter

### One inductor, shared by taking turns

**Field marks.** One inductor beside a power chip with several switching outputs. On the MAX77654, a 3.3 nF capacitor from its BST pin to LXB as well.

**Look-alike.** One buck followed by LDOs, which also gets by with one inductor. The MAX77654’s datasheet compares itself with that, and claims better total system efficiency.

**Range.** Each output 0.8 to 5.5 V. Peak currents of 0.335, 0.5, 0.75 or 1 A. A 1.0 to 2.2 µH inductor, 1.5 µH for most designs. 1 µA of quiescent current per output.

**Habitat.** Hearables, fitness monitors, wearables and sensor nodes, in ADI’s list.

## Taking turns

*One rail at a time*

Each rail has its own comparator, and asks when it falls to its set point. Rails that don’t ask are skipped. When none ask, the inductor rests.

A rail that asks while the inductor is busy waits its turn. At 6 µs the 1.8 V rail asks and gets a packet. The 3.3 V rail asks 0.1 µs later and waits 0.7 µs for that packet to end. When several rails are waiting, they’re served in turn, starting after the rail served last.

A heavier load asks more often. After the 1.8 V load triples at 12 µs, that rail gets 6 packets in the next 22 µs. No rail falls more than 2.3 mV below its set point.

**Fig. 22g: The opening’s packets, by rail.**

![Fig. 22g: The opening’s packets, by rail](https://nearfieldnotes.com/notes/simo/fig-22g.png)

*What the figure shows:* The opening's 15 packets split by rail over 34 µs: 6 to 1.1 V, 7 to 1.8 V and 2 to 3.3 V. The 1.8 V packets come more often after its load steps to 60 mA at 12 µs. The first 3.3 V packet waits 0.7 µs for a 1.8 V packet that asked first.

The same 15 packets as the opening figure. The 1.1 and 1.8 V packets peak at 0.5 A, the 3.3 V ones at 1 A.

**The numbers**

| Rail | Packets before 12 µs | After | Lowest before | Lowest after |
|---|---|---|---|---|
| 1.1 V | 2 | 4 | -0.05 mV | -0.61 mV |
| 1.8 V | 1 | 6 | -0.02 mV | -2.34 mV |
| 3.3 V | 1 | 1 | -1.14 mV | -1.06 mV |

## Ripple

*What each packet lifts*

Each packet lifts its rail by its charge over the rail’s capacitor. The datasheet sizes that capacitor from it, in its Eq. 3. Turned round, for buck-boost:

`ΔV = (L · I_pk^2)/(2 · V_out · C)`

where ΔV: ripple, the lift from one packet (mV); L: inductance (µH); I_pk: peak current (A); V_out: the rail’s voltage (V); C: the rail’s capacitance (µF).

`= (1.5 µH × (0.5 A)^2)/(2 × 1.1 V × 10 µF) = 17.0 mV`

One packet in the sim lifts the rail 15.2 mV, since resistance took some of its charge. In buck mode the fill’s charge adds to the lift. Either way, a bigger capacitor or a lower peak current makes it smaller.

Fig. 22h sets the formula against the sim’s ripple, from each rail’s lowest point to its highest. They land within 3 mV on every rail. Resistance trims each packet, and the load keeps drawing while one is under way. A rail that has to wait sags further. In buck-boost the 1.1 V rail at one point waits behind two other packets and falls 5.5 mV below its set point. That puts its ripple above the formula.

The capacitors here are 10 µF. A 0402 ceramic gives less than that at its rail’s voltage, through [DC bias](https://nearfieldnotes.com/glossary/#g-dc-bias), and the ripple grows to match. [Note 13](https://nearfieldnotes.com/notes/capacitor-types/) compares the kinds.

**Fig. 22h: Ripple on each rail.**

![Fig. 22h: Ripple on each rail](https://nearfieldnotes.com/notes/simo/fig-22h.png)

*What the figure shows:* Ripple on each rail, the datasheet's formula against the sim. 1.1 V buck mode, formula 24.3 mV, simulated 21.6 mV. 1.1 V buck-boost, formula 17.0 mV, simulated 17.5 mV. 1.8 V buck mode, formula 20.3 mV, simulated 20.3 mV. 1.8 V buck-boost, formula 10.4 mV, simulated 10.4 mV. 3.3 V buck-boost, formula 22.7 mV, simulated 20.6 mV.

10 µF on each rail. The 3.3 V rail has no buck-mode row, since the cell isn’t 0.7 V above it.

**The numbers**

| Rail | Mode | Formula | Simulated | Packets a second |
|---|---|---|---|---|
| 1.1 V | buck mode | 24.3 mV | 21.6 mV | 177,000 |
| 1.1 V | buck-boost | 17.0 mV | 17.5 mV | 260,000 |
| 1.8 V | buck mode | 20.3 mV | 20.3 mV | 100,000 |
| 1.8 V | buck-boost | 10.4 mV | 10.4 mV | 207,000 |
| 3.3 V | buck-boost | 22.7 mV | 20.6 mV | 47,000 |

## What it costs

*Busy time*

The rails [share the inductor’s time](https://nearfieldnotes.com/glossary/#g-utilization). At this watch’s loads it’s busy 28% of the time. With every rail in buck-boost it would be busy 38%, since each packet then carries less.

Load the 1.1 V rail harder and its packets come closer together, until they run back to back. Run the datasheet’s way, it holds up to 210 mA. All buck-boost, it holds up to 150 mA. Past that it can’t keep up, and its voltage falls away.

**Fig. 22i: Busy time against load.**

![Fig. 22i: Busy time against load](https://nearfieldnotes.com/notes/simo/fig-22i.png)

*What the figure shows:* How much of the time the inductor is busy, against the 1.1 V rail's load, with the 1.8 V and 3.3 V rails at 20 and 10 mA. As the datasheet runs it, busy 12% at no load, 28% at 40 mA, and 97% at 210 mA, the last load the 1.1 V rail holds. All buck-boost it reaches 98% at 150 mA and holds no more.

The 1.8 and 3.3 V rails at 20 and 10 mA throughout. Each line ends at the last load the 1.1 V rail holds within 60 mV of its set point.

**Every load**

| 1.1 V load | As the datasheet runs it | All buck-boost |
|---|---|---|
| 0 mA | 12% | 16% |
| 10 mA | 16% | 22% |
| 20 mA | 20% | 27% |
| 30 mA | 24% | 33% |
| 40 mA | 28% | 38% |
| 50 mA | 32% | 44% |
| 60 mA | 36% | 49% |
| 70 mA | 40% | 55% |
| 80 mA | 45% | 60% |
| 90 mA | 48% | 66% |
| 100 mA | 53% | 71% |
| 110 mA | 57% | 77% |
| 120 mA | 61% | 82% |
| 130 mA | 65% | 88% |
| 140 mA | 69% | 93% |
| 150 mA | 73% | 98% |
| 160 mA | 77% | can't keep up |
| 170 mA | 81% | can't keep up |
| 180 mA | 85% | can't keep up |
| 190 mA | 89% | can't keep up |
| 200 mA | 93% | can't keep up |
| 210 mA | 97% | can't keep up |
| 220 mA | can't keep up | can't keep up |
| 230 mA | can't keep up | can't keep up |
| 240 mA | can't keep up | can't keep up |
| 250 mA | can't keep up | can't keep up |

Utilization

### The share of time the inductor is busy

**Field marks.** Packets back to back with no rest between them, and one rail sagging under load while the others hold.

**Look-alike.** A buck’s [duty cycle](https://nearfieldnotes.com/glossary/#g-duty-cycle). That is one switch’s share of a fixed period. A SIMO has no fixed period, and its busy time counts every packet to every rail.

**Range.** 28% at this watch’s loads, and 97% at the most its 1.1 V rail holds. Over 100%, the datasheet says, the outputs droop.

**Habitat.** Every SIMO. The MAX77654’s design example works it out from each rail’s load, peak current and voltages, and keeps it under 80%.

The datasheet’s fix for too much busy time is a higher peak current. Each packet then carries more, so fewer are needed, but the ripple grows with the square of the peak current.

The switching frequency has no fixed value either. It follows the loads. In the sim, the 1.1 V rail at 210 mA takes 910,000 packets a second. The datasheet gives about 327 kHz for its own 3.7 to 1.2 V example, and as low as 10 Hz with no load.

## Which way to make the rails

*A key*

Key to a small board’s rails

### What should make three rails from one cell?

1. 1a Every rail sits close under the cell, or draws microamps LDOs
2. 1b A rail sits far under the cell and draws milliamps Go to 2
3. 2a There’s room for an inductor on each rail A buck for each
4. 2b There’s room for one inductor Go to 3
5. 3a Its packets keep it busy under 80%One SIMO
6. 3b They’d keep it busy over 80%One SIMO, higher peak currents

This watch’s answer is 3a: its inductor is busy 28% of the time.

## Load the rails yourself

*100 runs, every one drawn*

Set each rail’s load. Each hump is one packet of inductor current to that rail.

A heavier load asks more often and takes more of the inductor’s time. Load the rails heavily and the inductor runs out of time. The 1.1 V rail falls behind first.

**Note 22’s explorer: Packets on each rail.**

![Note 22’s explorer: Packets on each rail](https://nearfieldnotes.com/notes/simo/fig-explore.png)

*What the figure shows:* 40 µs of packets on each rail for the chosen loads, as humps of inductor current.

Run the datasheet’s way, with 10 µF on each rail. Each run lasts 400 µs, and the strip is its last 40 µs. Busy time is over its last 300 µs, ripple over its last 100 µs. A rail that falls 60 mV below its set point can’t keep up.

**Every case, as a table**

| 1.1 V load | 1.8 V load | 3.3 V load | Busy | 1.1 V ripple | 1.8 V ripple | 3.3 V ripple |
|---|---|---|---|---|---|---|
| 10 mA | 5 mA | 5 mA | 8% | 21.8 mV | 19.9 mV | 20.8 mV |
| 10 mA | 5 mA | 10 mA | 10% | 21.8 mV | 19.9 mV | 20.6 mV |
| 10 mA | 5 mA | 20 mA | 14% | 21.8 mV | 19.9 mV | 20.2 mV |
| 10 mA | 5 mA | 40 mA | 22% | 22.5 mV | 19.9 mV | 19.4 mV |
| 10 mA | 20 mA | 5 mA | 14% | 21.8 mV | 18.7 mV | 20.8 mV |
| 10 mA | 20 mA | 10 mA | 16% | 21.9 mV | 18.7 mV | 20.6 mV |
| 10 mA | 20 mA | 20 mA | 20% | 21.9 mV | 18.7 mV | 20.2 mV |
| 10 mA | 20 mA | 40 mA | 28% | 22.5 mV | 19.1 mV | 19.4 mV |
| 10 mA | 40 mA | 5 mA | 22% | 22.0 mV | 17.2 mV | 20.8 mV |
| 10 mA | 40 mA | 10 mA | 24% | 22.0 mV | 17.2 mV | 20.6 mV |
| 10 mA | 40 mA | 20 mA | 28% | 22.0 mV | 17.2 mV | 20.2 mV |
| 10 mA | 40 mA | 40 mA | 36% | 22.5 mV | 17.7 mV | 19.4 mV |
| 10 mA | 60 mA | 5 mA | 30% | 22.6 mV | 20.9 mV | 20.8 mV |
| 10 mA | 60 mA | 10 mA | 32% | 22.6 mV | 16.6 mV | 21.4 mV |
| 10 mA | 60 mA | 20 mA | 36% | 22.6 mV | 19.3 mV | 21.6 mV |
| 10 mA | 60 mA | 40 mA | 44% | 22.5 mV | 20.2 mV | 21.7 mV |
| 10 mA | 80 mA | 5 mA | 38% | 22.2 mV | 15.2 mV | 20.9 mV |
| 10 mA | 80 mA | 10 mA | 40% | 22.2 mV | 15.4 mV | 21.4 mV |
| 10 mA | 80 mA | 20 mA | 44% | 22.2 mV | 19.1 mV | 21.6 mV |
| 10 mA | 80 mA | 40 mA | 52% | 22.9 mV | 20.3 mV | 22.3 mV |
| 40 mA | 5 mA | 5 mA | 20% | 19.2 mV | 20.1 mV | 20.8 mV |
| 40 mA | 5 mA | 10 mA | 22% | 21.7 mV | 20.1 mV | 20.6 mV |
| 40 mA | 5 mA | 20 mA | 26% | 21.9 mV | 20.1 mV | 20.8 mV |
| 40 mA | 5 mA | 40 mA | 34% | 22.2 mV | 20.1 mV | 21.8 mV |
| 40 mA | 20 mA | 5 mA | 26% | 19.8 mV | 20.3 mV | 20.8 mV |
| 40 mA | 20 mA | 10 mA | 28% | 21.6 mV | 20.3 mV | 20.6 mV |
| 40 mA | 20 mA | 20 mA | 32% | 21.8 mV | 20.4 mV | 20.8 mV |
| 40 mA | 20 mA | 40 mA | 41% | 22.1 mV | 20.5 mV | 22.0 mV |
| 40 mA | 40 mA | 5 mA | 34% | 20.7 mV | 20.9 mV | 20.8 mV |
| 40 mA | 40 mA | 10 mA | 36% | 22.4 mV | 20.3 mV | 20.6 mV |
| 40 mA | 40 mA | 20 mA | 40% | 22.3 mV | 20.4 mV | 20.9 mV |
| 40 mA | 40 mA | 40 mA | 48% | 22.0 mV | 20.7 mV | 22.1 mV |
| 40 mA | 60 mA | 5 mA | 42% | 21.4 mV | 20.7 mV | 21.2 mV |
| 40 mA | 60 mA | 10 mA | 44% | 21.3 mV | 22.2 mV | 21.4 mV |
| 40 mA | 60 mA | 20 mA | 48% | 21.6 mV | 22.7 mV | 21.5 mV |
| 40 mA | 60 mA | 40 mA | 57% | 22.4 mV | 23.7 mV | 22.1 mV |
| 40 mA | 80 mA | 5 mA | 50% | 21.3 mV | 21.5 mV | 20.9 mV |
| 40 mA | 80 mA | 10 mA | 52% | 24.3 mV | 21.6 mV | 21.4 mV |
| 40 mA | 80 mA | 20 mA | 56% | 24.2 mV | 21.1 mV | 22.6 mV |
| 40 mA | 80 mA | 40 mA | 64% | 23.9 mV | 21.6 mV | 25.4 mV |
| 80 mA | 5 mA | 5 mA | 36% | 16.0 mV | 20.3 mV | 21.1 mV |
| 80 mA | 5 mA | 10 mA | 38% | 22.2 mV | 20.2 mV | 21.1 mV |
| 80 mA | 5 mA | 20 mA | 42% | 22.5 mV | 20.2 mV | 21.6 mV |
| 80 mA | 5 mA | 40 mA | 51% | 22.3 mV | 20.2 mV | 22.8 mV |
| 80 mA | 20 mA | 5 mA | 43% | 21.0 mV | 20.5 mV | 21.1 mV |
| 80 mA | 20 mA | 10 mA | 45% | 21.9 mV | 20.5 mV | 21.1 mV |
| 80 mA | 20 mA | 20 mA | 49% | 22.4 mV | 20.3 mV | 21.7 mV |
| 80 mA | 20 mA | 40 mA | 57% | 22.1 mV | 21.0 mV | 23.0 mV |
| 80 mA | 40 mA | 5 mA | 50% | 22.0 mV | 20.7 mV | 21.1 mV |
| 80 mA | 40 mA | 10 mA | 52% | 21.8 mV | 20.8 mV | 21.2 mV |
| 80 mA | 40 mA | 20 mA | 56% | 22.5 mV | 20.3 mV | 21.7 mV |
| 80 mA | 40 mA | 40 mA | 65% | 22.8 mV | 21.8 mV | 22.6 mV |
| 80 mA | 60 mA | 5 mA | 58% | 21.6 mV | 20.9 mV | 21.5 mV |
| 80 mA | 60 mA | 10 mA | 60% | 21.3 mV | 22.4 mV | 22.0 mV |
| 80 mA | 60 mA | 20 mA | 64% | 21.7 mV | 23.0 mV | 22.9 mV |
| 80 mA | 60 mA | 40 mA | 73% | 28.7 mV | 26.3 mV | 24.8 mV |
| 80 mA | 80 mA | 5 mA | 66% | 24.8 mV | 21.3 mV | 20.9 mV |
| 80 mA | 80 mA | 10 mA | 68% | 22.3 mV | 21.5 mV | 21.5 mV |
| 80 mA | 80 mA | 20 mA | 72% | 22.3 mV | 21.4 mV | 23.5 mV |
| 80 mA | 80 mA | 40 mA | 81% | 25.2 mV | 22.7 mV | 25.6 mV |
| 120 mA | 5 mA | 5 mA | 52% | 19.6 mV | 20.1 mV | 21.0 mV |
| 120 mA | 5 mA | 10 mA | 54% | 19.4 mV | 20.1 mV | 21.0 mV |
| 120 mA | 5 mA | 20 mA | 58% | 23.4 mV | 20.2 mV | 21.8 mV |
| 120 mA | 5 mA | 40 mA | 67% | 22.5 mV | 20.2 mV | 23.0 mV |
| 120 mA | 20 mA | 5 mA | 59% | 21.7 mV | 19.7 mV | 21.0 mV |
| 120 mA | 20 mA | 10 mA | 61% | 21.4 mV | 19.7 mV | 21.1 mV |
| 120 mA | 20 mA | 20 mA | 65% | 22.6 mV | 19.8 mV | 21.9 mV |
| 120 mA | 20 mA | 40 mA | 73% | 22.9 mV | 21.0 mV | 23.0 mV |
| 120 mA | 40 mA | 5 mA | 67% | 20.3 mV | 19.6 mV | 21.0 mV |
| 120 mA | 40 mA | 10 mA | 69% | 19.8 mV | 19.7 mV | 21.1 mV |
| 120 mA | 40 mA | 20 mA | 73% | 21.6 mV | 20.5 mV | 22.1 mV |
| 120 mA | 40 mA | 40 mA | 81% | 22.6 mV | 21.9 mV | 26.0 mV |
| 120 mA | 60 mA | 5 mA | 74% | 22.4 mV | 21.3 mV | 21.1 mV |
| 120 mA | 60 mA | 10 mA | 76% | 22.6 mV | 21.2 mV | 21.3 mV |
| 120 mA | 60 mA | 20 mA | 80% | 24.7 mV | 25.4 mV | 22.1 mV |
| 120 mA | 60 mA | 40 mA | 89% | 31.9 mV | 22.6 mV | 23.2 mV |
| 120 mA | 80 mA | 5 mA | 83% | 21.4 mV | 22.2 mV | 21.1 mV |
| 120 mA | 80 mA | 10 mA | 85% | 21.4 mV | 22.2 mV | 21.4 mV |
| 120 mA | 80 mA | 20 mA | 89% | 32.8 mV | 28.2 mV | 23.4 mV |
| 120 mA | 80 mA | 40 mA | 97% | 29.1 mV | 28.6 mV | 25.9 mV |
| 160 mA | 5 mA | 5 mA | 69% | 22.7 mV | 20.0 mV | 21.2 mV |
| 160 mA | 5 mA | 10 mA | 71% | 22.4 mV | 20.0 mV | 21.3 mV |
| 160 mA | 5 mA | 20 mA | 75% | 22.3 mV | 20.0 mV | 22.0 mV |
| 160 mA | 5 mA | 40 mA | 83% | 23.8 mV | 20.0 mV | 23.0 mV |
| 160 mA | 20 mA | 5 mA | 75% | 21.3 mV | 20.5 mV | 21.2 mV |
| 160 mA | 20 mA | 10 mA | 77% | 22.2 mV | 20.4 mV | 21.4 mV |
| 160 mA | 20 mA | 20 mA | 81% | 29.1 mV | 20.4 mV | 22.0 mV |
| 160 mA | 20 mA | 40 mA | 89% | 29.1 mV | 21.1 mV | 23.0 mV |
| 160 mA | 40 mA | 5 mA | 83% | 23.4 mV | 20.4 mV | 21.0 mV |
| 160 mA | 40 mA | 10 mA | 85% | 29.1 mV | 20.9 mV | 21.0 mV |
| 160 mA | 40 mA | 20 mA | 89% | 29.1 mV | 20.8 mV | 21.5 mV |
| 160 mA | 40 mA | 40 mA | 97% | 29.1 mV | 24.3 mV | 22.9 mV |
| 160 mA | 60 mA | 5 mA | 91% | 29.1 mV | 21.2 mV | 21.5 mV |
| 160 mA | 60 mA | 10 mA | 93% | 29.1 mV | 24.3 mV | 21.9 mV |
| 160 mA | 60 mA | 20 mA | 97% | 34.1 mV | 26.2 mV | 22.8 mV |
| 160 mA | 60 mA | 40 mA | 100% | Can't keep up | 26.5 mV | 27.5 mV |
| 160 mA | 80 mA | 5 mA | 99% | 38.3 mV | 22.3 mV | 21.2 mV |
| 160 mA | 80 mA | 10 mA | 100% | Can't keep up | 21.6 mV | 22.2 mV |
| 160 mA | 80 mA | 20 mA | 100% | Can't keep up | 30.2 mV | 24.0 mV |
| 160 mA | 80 mA | 40 mA | 100% | Can't keep up | Can't keep up | 32.3 mV |

#### Conditions

- **Cell and loads:** 300 mAh at a steady 3.7 V. 1.1 V at 40 mA, 1.8 V at 20 mA, 3.3 V at 10 mA, 113 mW in all.
- **LDOs:** Three TI TPS7A02. The cell supplies the loads’ current plus 25 nA each.
- **Bucks:** A TI TPS62841 on the 1.1 V rail, and a TI TPS62840 on each of the 1.8 and 3.3 V rails. The TPS62841 covers 0.8 to 1.55 V, and the TPS62840 1.8 to 3.3 V. Each has a 2.2 µH Murata DFE201612E, 2.0 by 1.6 mm. Efficiency at each rail’s load read off the datasheet’s curves at 3.6 V in: 86.5, 90 and 93%. The 1.1 V value lies between its 0.8 and 1.2 V curves.
- **SIMO:** ADI MAX77654 with one 1.5 µH inductor and 10 µF on each rail. Efficiency read off the datasheet’s curves at 3.7 V in, with 1.1 and 1.8 V in buck mode at a 0.5 A peak and 3.3 V in buck-boost at 1 A: 86.8, 91 and 85%. Those curves use a 2.2 µH inductor. Plus 37 µA of bias current, the project’s allowance. The datasheet gives 28 µA for its main bias. The difference moves battery life by seconds.
- **Board:** A 22 mm KiCad layout. Each way’s area is the sum of its parts’ courtyards. The room for power was sized to the SIMO’s own layout. The bucks are laid out as tightly, from the room’s top-left corner. The heart-rate sensor, fuel gauge and haptic driver are stand-ins at those parts’ usual sizes.
- **Packets:** A cycle-by-cycle model in 1 ns steps. An ideal comparator on each rail, turns taken from the rail served last, a rest when none ask. Peak currents 0.5, 0.5 and 1 A. Switches 0.15 Ω and inductor 80 mΩ, both estimated. Loads are constant currents.
- **Renders:** Blender, from the KiCad board and its parts’ published sizes.

Simulated and read from datasheets only. Nothing on this page has been measured on a board yet.

#### Limits

- The cell is held at 3.7 V. A real cell falls as it empties, and near 3.3 V a 3.3 V LDO or buck can no longer hold its rail.
- Battery life rests on efficiencies read off curves by eye, at one input voltage. The 12 minutes between the bucks and the SIMO come to about two points of efficiency.
- The model’s resistances are estimates, so its own efficiency counts conduction loss only: 90% the datasheet’s way, 87% all buck-boost. No figure here uses it.
- The comparators are ideal, with no delay or offset, and the model has no fault handling. Past its limit a real part may flag a fault.
- The capacitors are an ideal 10 µF, with no DC bias loss and no resistance.

#### Files

- sim/simo.py The packet model
- sim/site.py Buck mode, and every run on this page
- sim/compare.py Battery life from the datasheet curves
- layout/board.py The watch board in KiCad, all three ways
- render/site.py The board renders on white

#### Sources

- [ADI, MAX77654 datasheet, 19-100560 Rev 2](https://www.analog.com/media/en/technical-documentation/data-sheets/MAX77654.pdf)
- [TI, TPS62840 datasheet, SLVSEC6D](https://www.ti.com/lit/ds/symlink/tps62840.pdf)
- [TI, TPS7A02 datasheet, SBVS277C](https://www.ti.com/lit/ds/symlink/tps7a02.pdf)
- [Murata, DFE201612E-2R2M data sheet](https://www.murata.com/en-us/products/productdetail?partno=DFE201612E-2R2M%23)

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Source: https://nearfieldnotes.com/notes/simo/

Cite as: Michael Walker, “SIMO converters”, Near Field Notes, Note 22, Rev 1, 6 Nov 2026. https://nearfieldnotes.com/notes/simo/

Figures: free to reuse under CC BY 4.0 (https://nearfieldnotes.com/license/). Each one's image is linked under its name. Credit Michael Walker, Near Field Notes, and link the figure: this address plus #fig- and its number, like https://nearfieldnotes.com/notes/simo/#fig-22.