HIP4086, HIP4086A
11
FN4220.11
January 12, 2017
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Charge Pump
The internal charge pump of the HIP4086/A is used to maintain
the bias on the boot capacitor 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 capacitor that is
consumed, when the xHO drivers source the gate charge to turn
on the high-side bridge FETs. The boot capacitors should be sized
so that they do not discharge excessively when sourcing the gate
charge. See “
Application Information” for methods to size the
boot capacitors.
The charge pump has sufficient capacity to source a worst-case
minimum of 40µA 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 capacitors.
Because the charge pump current is small, a gate-to-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 2
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 2
, 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.
If the gate-to-source resistor is removed (R
GS
is usually not
needed or recommended), then:
C
boot
= 0.33µF
These values of C
boot
will sustain the high-side driver bias during
Period with only a small amount of Ripple. But in the case of the
HIP4086, the charge pump reduces the value of C
boot
even
more. The specified charge pump current is a minimum of 40µA,
which is more than sufficient to source I
gate_leak
. Also, because
the specified charge pump current is in excess of what is needed
for I
HB
, the total charge required to be sourced by the boot
capacitor is shown by Equation 2.
Not only is the required boot capacitor smaller in value, there is
no restriction on the duration of Period.
TABLE 2.
V
DD
= 10V V
DD
can be any value between 7 and 15VDC.
V
HB
= V
DD
- 0.6V
= V
HO
High-side driver bias voltage (V
DD
- boot diode
voltage) referenced to V
HS
.
Period = 1ms This is the longest expected switching period.
I
HB
= 100µA Worst case high-side driver current when
xHO = high (this value is specified for V
DD
= 12V
but the error is not significant).
R
GS
= 100kΩ Gate-to-source resistor (usually not needed).
Ripple = 5% Desired ripple voltage on the boot capacitor
(larger ripple is not recommended).
I
gate_leak
= 100nA From the FET vendor’s datasheet.
Qgate80V = 64nC From Figure 22
.
Q
C
Q
gate80V
= Period (I
HB
V
HO
R
GS
I
gate_leak
+ )++
(EQ. 1)
C
boot
Q
C
= Ripple
VDD
C
boot
0.52F=
Q
C
Q
gate80V
= orC
boot
0.13F=
(EQ. 2)
FIGURE 22. TYPICAL GATE VOLTAGE vs GATE CHARGE
