TPS61170: how to choose Q1, Q2 in fig21

Hi Given,

Generally, for Q1 a integrated pair-BIJ is always recommended to achieve better matching for current mirror. Q2 can use a general BJT with enough ratings. It will be better if the base current is small. 

You may use the one that recommended in datasheet directly: Q1: BCM62B. Q2: MMBT3904TT1.

Regards

Lei

Hi Lei, 

can Q2 be replaced by MOSFET. To my understanding, Vbe of BJT is stable and Vgs of MOS is not stable, so Q2 cannot use MOS, right? Thanks. 

BRs

Given 

Hi Lei,

got it. 

To my understanding,  FB/R2=(-Vbe-Vout)/R1, then if using MOS, vbe is replaced by Vgs of MOS. Since Vgs is not stable, then MOS cannot be used, right? or we can still use MOS, but only add Vgs in the equation. Thanks. 

BRs

Given 

Hi Given,

Yes if using MOS, vbe is replaced by Vgs. Compared with Vbe, Vgs is bigger and can't be ignored. So the Vout will be hard to adjusted to the wanted value accurately.

Regards

Lei