HIP4086A Ver la hoja de datos (PDF) - Renesas Electronics
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HIP4086A Datasheet PDF : 17 Pages
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HIP4086, HIP4086A
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.
VDD = 10V
VHB = VDD - 0.6V
= VHO
Period = 1ms
IHB= 100µA
RGS = 100kΩ
Ripple = 5%
TABLE 2.
VDD can be any value between 7 and 15VDC.
High-side driver bias voltage (VDD - boot diode
voltage) referenced to VHS.
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-to-source resistor (usually not needed).
Desired ripple voltage on the boot capacitor
(larger ripple is not recommended).
Igate_leak = 100nA
Qgate80V = 64nC
From the FET vendor’s datasheet.
From Figure 22.
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.
QC = Qgate80V + Period (IHB + VHO RGS + Igate_leak)
Cboot = QC RippleVDD
Cboot = 0.52F
(EQ. 1)
If the gate-to-source resistor is removed (RGS is usually not
needed or recommended), then:
Cboot = 0.33µF
These values of Cboot 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 Cboot even
more. The specified charge pump current is a minimum of 40µA,
which is more than sufficient to source Igate_leak. Also, because
the specified charge pump current is in excess of what is needed
for IHB, the total charge required to be sourced by the boot
capacitor is shown by Equation 2.
QC = Qgate80Vor Cboot = 0.13F
(EQ. 2)
Not only is the required boot capacitor smaller in value, there is
no restriction on the duration of Period.
FIGURE 22. TYPICAL GATE VOLTAGE vs GATE CHARGE
FN4220 Rev 1.00
January 12, 2017
Page 11 of 17
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