DD

DH

OHH

HOH

Gate

GFET_Int

V

V

10 V 1 V

I

0.48 A

R

R

R

1.2 V / 100 mA

4.7

2.2

:

:

DD

DH

DBoot(pk)

Boot

V

V

10 V 1 V

I

4 A

R

2.2

|

:

Total

Boot

HB

Q

17.5 nC

C

7.6 nF

V

2.3 V

'

12

LM5109A

SNVS412C – APRIL 2006 – REVISED SEPTEMBER 2016

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Copyright © 2006–2016, Texas Instruments Incorporated

•

IHBS = HB to VSS Leakage current

•

DMax = Converter maximum duty cycle

•

IHB = HB Quiescent current

(2)

Therefore, the minimum CBoot must be:

(3)

In practice, the value of the CBoot capacitor must be greater than calculated to allow for situations where the

power stage may skip pulse due to load transients. TI recommends having enough margins and place the

bootstrap capacitor as close to the HB and HS pins as possible.

CBoot = 100 nF

(4)

As a general rule the local VDD bypass capacitor must be 10 times greater than the value of CBoot, as shown in

Equation 5.

CVDD = 1 µF

(5)

The bootstrap and bias capacitors must be ceramic types with X7R dielectric. The voltage rating must be twice

that of the maximum VDD considering capacitance tolerances once the devices have a DC bias voltage across

them and to ensure long-term reliability.

8.2.2.2 Select External Bootstrap Diode and Its Series Resistor

The bootstrap capacitor is charged by the VDD through the external bootstrap diode every cycle when low-side

MOSFET turns on. The charging of the capacitor involves high peak currents, and therefore transient power

dissipation in the bootstrap diode may be significant and the conduction loss also depends on its forward voltage

drop. Both the diode conduction losses and reverse recovery losses contribute to the total losses in the gate

driver circuit.

For the selection of external bootstrap diodes, see AN-1317 Selection of External Bootstrap Diode for LM510X

Devices (SNVSA083). Bootstrap resistor RBOOT is selected to reduce the inrush current in DBOOT and limit the

ramp up slew rate of voltage of VHB-HS during each switching cycle, especially when HS pin have excessive

negative transient voltage. RBOOT recommended value is between 2 Ω and 10 Ω depending on diode selection. A

current limiting resistor of 2.2 Ω is selected to limit inrush current of bootstrap diode, and the estimated peak

current on the DBoot is shown in Equation 6.

where

•

VDH is the bootstrap diode forward voltage drop

(6)

8.2.2.3 Selecting External Gate Driver Resistor

The external gate driver resistor, RGATE, is sized to reduce ringing caused by parasitic inductances and

capacitances and also to limit the current coming out of the gate driver.

Peak HO pullup current are calculated in Equation 7.

where

•

IOHH = Peak pullup current

•

VDH = Bootstrap diode forward voltage drop

•

RHOH = Gate driver internal HO pullup resistance, provide by driver data sheet directly or estimated from the

testing conditions, that is RHOH = VOHH / IHO

•

RGate = External gate drive resistance

•

RGFET_Int = MOSFET internal gate resistance, provided by transistor data sheet

(7)

Similarly, Peak HO pulldown current is shown in Equation 8.