Help me fix my power supply circuit based on LP2981

[saurav_sdpl]

I have designed a circuit which will supply power to the PCB, both with Adaptor or battery.

Adaptor input voltage is 4.5V(Vin)

Battery is 2X1.2V

But its not working, Plz verify if something i can do more in this

 

[Btrend]

Re: lp2981

I got questions about ur ckt,
1. what is the target output voltage ? 4.5V or 3.3V ?
2. The Vout of L6920 should be equipped with at least 47uF capacitor as the spec. suggested.
3. Do that ckt. automatic switch between Vin (4.5V) & Vout of L6920 ?
4. The label are not clear : BATT=VBATT ? and VBAT connect to where ?

 

[saurav_sdpl]

Re: lp2981

I am incorporating the switch circuit such that when 4.3V is not their, The input to L6920 is batery voltage.

This i am doing through MOSFET, with source connected to battery and drain to input of L6920.

So whether it will work??

Also how can i prevent current entering into LP2981 during power down mode from o/p capacitor, without giving a bypass diode,

Actually if i connect a diode in series at the o/p i can get the desired action but at the expense of diode drop(.3V or .7V)

Is their any other way.

 

[Btrend]

Re: lp2981

1. the switch MOS seems O.K.
2. but L6920 is a DC-DC boost , it need a loading capacitor 47uF to store energy from inductor, if the capcitor is too small the L6920 may not work properly.
3. also in ur design, L6920 output 3.3V only, so if u use Vin=4.5V to VBATT, the boost will not work, cause input voltage is larger than output voltage.
4. I don't see any LP2981 ckt in ur attachment ?