Contents
What happens when USB voltage is not present?
When USB voltage is absent Q3 = P Channel MOSFET is turned on by R4, feeding battery voltage to LDO and thence to Vout. When USB voltage is present Q1 is trurned on by R2/R3 and this turns on Q2 (usually held off by R1) which clamps Q3 gate high turning it off, thus disabling battery feed.
How to reduce battery load when using USB?
USB power feeds via D1 either via Ja and LDo or Jb as above. Changed R1, R4, R5 to nominal 1 megohm each to reduce battery load when USB in use. A small MOSFET for Q2 and/or some more thinking will reduce required standby current.
What happens when battery is unplugged from USB?
Depending on =FET gate threshold it MAY take a few 10ths of a secind for battery to turn on when USB is unplugged. This could dropout powered cct unless a large enough output cap was provided. At R4 = 1m the time constant is about 10 milliseconds and a “usual” sort of cap on output rail would suffice.
When to turn off synchronous rectification in USB?
The circuit can then contain detection of which power is being used and turn on the appropriate FET to short accross that diode. You have to keep monitoring the two input voltages and turn off the FET when things change. In switching power supplies, this is called synchronous rectification.
How is USB power fed to the diode?
USB power is fed to load via Schottky diode D1. Power may be fed via Ja and the LDO regulator or via JB as desired. If feed via JB is used the LDO must survive with external voltage on its output when it has no input.
How is USB power fed to the battery?
It can be surprising how much difference a little more information can make to the solution of a problem. Battery feed at top. USB power feed at bottom. USB power is fed to load via Schottky diode D1. Power may be fed via Ja and the LDO regulator or via JB as desired.