HIP4086AABZ INTERSIL [Intersil Corporation], HIP4086AABZ Datasheet - Page 11

• Manufacturer Part Number: HIP4086AABZ
• Description: 80V, 500mA, 3-Phase MOSFET Driver 1.25A peak turn-off current
• Manufacturer: INTERSIL [Intersil Corporation]

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.
V
V
= V
Period = 1ms
I
R
Ripple = 5%
I
Qgate80V = 64nC
HB
gate_leak
DD
HB
GS
HO
= 100µA
= V
= 10V
= 100kΩ
DD
= 100nA
- 0.6V
V
High side driver bias voltage (V
voltage) referenced to V
This is the longest expected switching period
Worst case high side driver current when
xHO = high (this value is specified for VDD = 12V
but the error is not significant)
Gate-source resistor (usually not needed)
Desired ripple voltage on the boot cap (larger
ripple is not recommended)
From the FET vendor’s datasheet
From Figure L
DD
can be any value between 7 and 15VDC
TABLE 1.
11
HS
HIP4086, HIP4086A
DD
- boot diode
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
needed or recommended), then:
C
These values of C
Period with only a small amount of Ripple. But in the case of the
HIP4086, the charge pump reduces the value of C
more. The specified charge pump current is a minimum of 50µA
which is more than sufficient to source I
the specified charge pump current is in excess of what is needed
for I
capacitor is just
Not only is the required boot cap smaller in value, there is no
restriction on the duration of Period.
Q
C
C
Q
boot
boot
boot
C
C
=
=
HB
Q
Q
=
= 0.33µF
=
gate80V
, the total charge required to be sourced by the boot
gate80V
FIGURE 21. TYPICAL GATE VOLTAGE vs GATE CHARGE
Q
0.52μF
C
⁄
(
Ripple ∗ VDD
or C
+
Period (I
boot
boot
will sustain the high side driver bias during
=
×
0.13μF
)
HB
+
V
HO
⁄
R
GS
gate_leak
+
I
GS
gate_leak
is usually not
. Also, because
boot
)
even
June 1, 2011
FN4220.7
(EQ. 2)
(EQ. 1)