I'm designing a power supply for a system which requires a 3.3V logic power rail, driven by a user-selected battery.
My requirements are as follows:
- Input voltage between 7V and 18V - usual batteries are 7.4 / 11.1 / 14.8V LiPo, or 8.4 / 9.6V NiMH. Some users may choose a 9.9V LiFe PO4 battery, but this is unusual.
- 3.3V output (AP1509-33's tolerances are acceptable).
- 1500mA continuous, 1800mA peak.
- Quiescent current < 25mA.
- Reverse voltage protection.
- Overvoltage protection.
- Inrush current limiting on the 3.3V side (due to C4 on design below).
- Delay-start to ensure stable voltage is available on startup.
- Battery voltage sensing (via microcontroller ADC).
- Regulated voltage sensing (via INA219 power monitor IC).
- Overcurrent and overheat protection with redundancy.
- No replaceable fuses - polyfuse only (although many users will install a standard inline cartridge fuse anyway).
- Small design footprint.
Here are my schematics:
- U2: AP1509-33 150KHz, 2A PWM BUCK DC/DC CONVERTER (3.3V Fixed)
- Q1: FDD4141 P-Channel MOSFET: -40V, -50A, 12.3mΩ
- D1: 1N5822 Schottky Diode
- D3, D4: 1N47xx Series Zener Diodes
- U5, U12: INA219 Zero-Drift, Bidirectional Current/Power Monitor With I2C Interface
In the schematics above the ground net is directly connected to battery negative.
Both F1 and F2 are polyfuses. F1 has a trip time of around 4.0s at 750mA, dropping rapidly at higher currents. F2 is a very fast polyfuse with a trip time of one second at 2000mA. I can't find the exact datasheets or part numbers for them right now.
I used a P-channel MOSFET for reverse voltage protection rather than a diode for efficiency. The FD4141 was selected simply because it's the cheapest MOSFET I found that has appropriate voltage and current ratings and a low Rds(on).
D4 is intended to shunt current once BATT_PP exceeds 20V, quickly tripping F1. This should cover overvoltage scenarios.
The voltage divider and zener arrangement (R2, R3, D3) produces a sense voltage which directly drives an ADC pin on a microcontroller. At BATT_PP = 20V, D4 starts to conduct and VSENSE never exceeds around 3.2V. Below this there is a roughly linear relationship between BATT_PP and VSENSE. I'm using this as a cheap initial "voltage ok" check before powering on peripheral devices.
As per the datasheet, the arrangement of C5 and R1 on the shutdown (SD) pin of the AP1509-33 causes a soft-start and should limit inrush current on the 3.3V side.
L1, C4, and C9 are chosen based on the datasheet and standard advice on power supply design. D1 was chosen based on some other circuits I found online, as the datasheet was not very helpful in this regard.
The remainder of the circuit shown is simply a pair of INA219 power monitor ICs which provide more accurate monitoring of the system voltages. An alert is sounded if expected ranges are exceeded. My design does not currently allow for automatic shutdown of the power supply, although the AP1509 itself has overheat protection built in.
Does this design look ok? Do my decisions sound sane? Are there any improvements you might make?
Some errors spotted by a friend on Twitter:
- The battery voltage sensing resistor (R9) should come after Q1, not before it, because the INA219 doesn't support reverse voltage sensing.
- D4's surge current limit is 250mA, which is insufficient. I'll replace it with something with a 2A+ current rating.
- F1 and F2 aren't 0805 package, they're 1210.
They also pointed out that it's probably worth using tantalum caps for C4 and C9 due to their lower ESR. This seems sensible.