I wanted to know that LP5907 LDO output capacitance value can be increased to what level. I want power supply backup or bulk cap at the output so that it can supply to microcontroller 3.3V for 300ms the 6mA current after input voltage is dropped below threshold. Also is there any other converter/LDO which can support the backup time of 300ms and lower on time for conversion.?
Hi Vishal,
You have a couple of options. Because you have a hold up requirement when the source supply goes away, you have to somehow block reverse current from flowing back through the LDO to Vin. You can either use an LDO with inherent reverse current protection to act as this block, or you can add a series diode to Vin. The series diode approach will open up your LDO options to just about everything, while the reverse current protection will narrow your options to just those LDO's with this built in feature.
LDO's with reverse current protection that may work for you: LP5910 (if Vin < 3.6V) or LP5912 (if you need more Vin).
I'm assuming you want the LDO with reverse current protection because your load is high (250mA) and you may not have the voltage to drop across the series diode in front of the LDO. Thus, let's assume you'll choose an LDO with reverse current protection. To start fast but still deliver the 6mA that you need during a brownout condition, you can add a series resistor with an anti-parallel diode (Rb and D2 in the schematic below). The series resistor will limit the current flowing through Cb (or trickle charge Cb). But when the load must be powered by Cb, D2 will short out Rb. You'll have to decide what time constant works for you. If 6mA through D2 drops 0.1V, then you have 0.2V to hold up in 300ms at 6mA load. Thus Cb > 9mF, and if R=100 ohms then 5 time constants = 4.5 seconds.
See below:
Cb = Big capacitor holding up Vout when Vin goes away
Using the first option, and with Cb =9mF, Rb=100 ohms, Vout =3.3V yields an initial peak current of 33 mA and declining across time.
Thanks,
Stephen
