AN4213 datasheet(23/42 Pages) STMICROELECTRONICS

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AN4213

DC/AC converter and lamp

The first parameter to take into account is the maximum voltage drop that we have to

guarantee when the high-side switch is in the on state. The maximum allowable voltage

drop (V

BOOT

) depends on the minimum gate drive voltage (for the high-side switch) to

maintain. If V

GSMIN

is the minimum gate-source voltage, the capacitor drop must be:

Equation 40

where:

•

is supply voltage of the gate driver [V];

•

is the bootstrap diode forward-voltage drop [V].

The value of the bootstrap capacitor is calculated by:

Equation 41

Where, Q

TOTAL

is the total amount of the charge supplied by the capacitor. The total charge

supplied by the bootstrap capacitor is calculated using

Equation 42:

Equation 42

Where:

•

GATE

is the total gate charge

•

LKCAP

is the bootstrap capacitor leakage current

•

LKGS

is the switch gate-source leakage current

•

QBS

is the bootstrap circuit quiescent current

•

is the bootstrap circuit leakage current

•

LKDIODE

is the bootstrap diode leakage current

•

is the high-side switch-on time

•

is the charge required by the internal level shifter, which is set to 3 nC for all HV

gate drivers.

The capacitor leakage current is important only if an electrolytic capacitor is used, otherwise

this can be neglected. Recommended values for the bootstrap capacitor are within the

range of 100 nF~570 nF, but the right value must be selected according to the application in

which the device is used. When the capacitor value is too large, the bootstrap charging time

slows and the low-side on-time might be not long enough to reach the bootstrap voltage. For

this project the selected value is 100 nF/400 V. The output drivers drive an external N-

channel Power MOSFET. The internal logic ensures a deadtime (typ 1.25 μs) to avoid cross-

conduction of the power devices. The selected power device is STL13N60M2.

GSMIN

BOOT

−

BOOT

TOTAL

BOOT

()

LKDIODE

QBS

LKGS

LKCAP

GATE

TOTAL

∗

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