Question about LTC7003 Bootstrapped Capacitor

Hello Kazu,

Kazu said:

Is the bootstrapped capacitor Cb required to be at least 10 times greater than Qg?

you cannot directly compare capacitance with charge.

Kazu said:

The formula on page 17 does not state 10x, which does not match the explanation on page 16, so let me check.

The formula means: the voltage over the bootstrap capacitor must not fall more than 1V to completely charge the gate of the FET. As a typical voltage over the bootstrap capacitor may be in the range of 10V, you have again the factor 10.

Kazu said:

Please tell me the internal operation of LTC7003 when the external NMOSFET is turned off due to small bootstrapped capacitance.

I believe it is operating as follows, is this correct?

  The reason why TG voltage drops is because bootstrapped capacitance < gate capacitance of external NMOSFET,

When the bootstrap capacitor is too small, the voltage over the BST-capacitor will drop too much when the FET is switched. And the LTC7003 monitors this voltage and switches the ouput off, when it is too small. The datasheet says:

"The LTC7003 additionally implements protection features
which prohibit TGUP being pulled to BST when V IN, V CC
or (VBST –V TS) are not within proper operating ranges."

Just for curiosity: why are you so much interested in minimal BST-capacitance? Normally you dimension the capacitor big enough with some reserve and don't try to hit the minimum possible value. 

best regards

Achim

Hello Kazu,

Kazu said:

Is the bootstrapped capacitor Cb required to be at least 10 times greater than Qg?

you cannot directly compare capacitance with charge.

Kazu said:

The formula on page 17 does not state 10x, which does not match the explanation on page 16, so let me check.

The formula means: the voltage over the bootstrap capacitor must not fall more than 1V to completely charge the gate of the FET. As a typical voltage over the bootstrap capacitor may be in the range of 10V, you have again the factor 10.

Kazu said:

Please tell me the internal operation of LTC7003 when the external NMOSFET is turned off due to small bootstrapped capacitance.

I believe it is operating as follows, is this correct?

  The reason why TG voltage drops is because bootstrapped capacitance < gate capacitance of external NMOSFET,

When the bootstrap capacitor is too small, the voltage over the BST-capacitor will drop too much when the FET is switched. And the LTC7003 monitors this voltage and switches the ouput off, when it is too small. The datasheet says:

"The LTC7003 additionally implements protection features
which prohibit TGUP being pulled to BST when V IN, V CC
or (VBST –V TS) are not within proper operating ranges."

Just for curiosity: why are you so much interested in minimal BST-capacitance? Normally you dimension the capacitor big enough with some reserve and don't try to hit the minimum possible value. 

best regards

Achim