HIP4086, HIP4086A

11

FN4220.10

March 27, 2015

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Charge Pump

The internal charge pump of the HIP4086/A is used to maintain

the bias on the boot cap for 100% duty cycle. There is no limit for

the duration of this period. The user must understand that this

charge pump is only intended to provide the static bias current of

the high-side drivers and the gate leakage current of the

high-side bridge FETs. It cannot provide in a reasonable time, the

majority of the charge on the boot cap that is consumed, when

the xHO drivers source the gate charge to turn on the high-side

bridge FETs. The boot caps should be sized so that they do not

discharge excessively when sourcing the gate charge. See

“Application Information” on page 11 for methods to size the

boot caps.

The charge pump has sufficient capacity to source a worst-case

minimum of 50µA to the external load. The gate leakage current

of most power MOSFETs is about 100nA so there is more than

sufficient current to maintain the charge on the boot caps.

Because the charge pump current is small, a gate-source resistor

on the high-side bridge FETs is not recommended. When

calculating the leakage load on the outputs of xHS, also include

the leakage current of the boot capacitor. This is rarely a problem

but it could be an issue with electrolytic capacitors at high

temperatures.

Application Information

Selecting the Boot Capacitor Value

The boot capacitor value is chosen not only to supply the internal

bias current of the high-side driver but also, and more

significantly, to provide the gate charge of the driven FET without

causing the boot voltage to sag excessively. In practice, the boot

capacitor should have a total charge that is about 20 times the

gate charge of the driven power FET for approximately a 5% drop

in voltage after charge has been transferred from the boot

capacitor to the gate capacitance.

The following parameters shown in Table 1 are required to

calculate the value of the boot capacitor for a specific amount of

voltage droop when using the HIP4086/A (no charge pump). In

Table 1, the values used are arbitrary. They should be changed to

comply with the actual application.

Equation 1 calculates the total charge required for the Period

duration. This equation assumes that all of the parameters are

constant during the Period duration. The error is insignificant if

Ripple is small.

needed or recommended), then:

Period with only a small amount of Ripple. But in the case of the

more. The specified charge pump current is a minimum of 50µA

the specified charge pump current is in excess of what is needed

capacitor is shown by Equation 2.

Not only is the required boot cap smaller in value, there is no

restriction on the duration of Period.

TABLE 1.

Period = 1ms

This is the longest expected switching period.

Worst case high-side driver current when

but the error is not significant).

Gate-source resistor (usually not needed).

Ripple = 5%

Desired ripple voltage on the boot cap (larger

ripple is not recommended).

From the FET vendor’s datasheet.

Qgate80V = 64nC

From Figure 21.

=



+



)

++

(EQ. 1)

=

RippleVDD





0.52

F

=

=

0.13

F

=

(EQ. 2)

FIGURE 21. TYPICAL GATE VOLTAGE vs GATE CHARGE