7
FN9157.6
September 11, 2015
In addition, more than 400mV hysteresis also incorporates
into the three-state shutdown window to eliminate PWM
input oscillations due to the capacitive load seen by the
PWM input through the body diode of the controller’s PWM
output when the power-up and/or power-down sequence of
bias supplies of the driver and PWM controller are required.
Power-On Reset (POR) Function
During initial start-up, the VCC voltage rise is monitored.
Once the rising VCC voltage exceeds 9.8V (typically),
operation of the driver is enabled and the PWM input signal
takes control of the gate drives. If VCC drops below the
falling threshold of 7.6V (typically), operation of the driver is
disabled.
Pre-POR Overvoltage Protection
For the ISL6594A, prior to VCC exceeding its POR level, the
upper gate is held low. For the ISL6594B, the upper gate
driver is powered from PVCC and will be held low when a
voltage of 2.75V or higher is present on PVCC as VCC
surpasses its POR threshold. For both devices, the lower
gate is controlled by the overvoltage protection circuits
during initial start-up. The PHASE is connected to the gate of
the low side MOSFET (LGATE), which provides some
protection to the microprocessor if the upper MOSFET(s) is
shorted during initial start-up. For complete protection, the
low side MOSFET should have a gate threshold well below
the maximum voltage rating of the load/microprocessor.
When VCC drops below its POR level, both gates pull low
and the Pre-POR overvoltage protection circuits are not
activated until VCC resets.
Internal Bootstrap Device
Both drivers feature an internal bootstrap Schottky diode.
Simply adding an external capacitor across the BOOT and
PHASE pins completes the bootstrap circuit. The bootstrap
function is also designed to prevent the bootstrap capacitor
from overcharging due to the large negative swing at the
trailing-edge of the PHASE node. This reduces voltage
stress on the boot to phase pins.
The bootstrap capacitor must have a maximum voltage
rating above UVCC + 5V and its capacitance value can be
chosen from Equation 1:
where Q
G1
is the amount of gate charge per upper MOSFET
at V
GS1
gate-source voltage and N
Q1
is the number of
control MOSFETs. The V
BOOT_CAP
term is defined as the
allowable droop in the rail of the upper gate drive.
As an example, suppose two IRLR7821 FETs are chosen as
the upper MOSFETs. The gate charge, Q
G
, from the data
sheet is 10nC at 4.5V (V
GS
) gate-source voltage. Then the
Q
GATE
is calculated to be 53nC for UVCC (i.e. PVCC in
ISL6594B, VCC in ISL6594A) = 12V. We will assume a
200mV droop in drive voltage over the PWM cycle. We find
that a bootstrap capacitance of at least 0.267F is required.
Gate Drive Voltage Versatility
The ISL6594A and ISL6594B provide the user flexibility in
choosing the gate drive voltage for efficiency optimization.
The ISL6594A upper gate drive is fixed to VCC [+12V], but
the lower drive rail can range from 12V down to 5V
depending on what voltage is applied to PVCC. The
ISL6594B ties the upper and lower drive rails together.
Simply applying a voltage from 5V up to 12V on PVCC sets
both gate drive rail voltages simultaneously.
Power Dissipation
Package power dissipation is mainly a function of the
switching frequency (f
SW
), the output drive impedance, the
external gate resistance, and the selected MOSFET’s
internal gate resistance and total gate charge. Calculating
the power dissipation in the driver for a desired application is
critical to ensure safe operation. Exceeding the maximum
allowable power dissipation level will push the IC beyond the
maximum recommended operating junction temperature of
+125°C. The maximum allowable IC power dissipation for
the SO8 package is approximately 800mW at room
temperature, while the power dissipation capacity in the DFN
package with an exposed heat escape pad is more than
1.5W. The DFN package is more suitable for high frequency
applications. See “Layout Considerations” on page 8 for
thermal transfer improvement suggestions. When designing
the driver into an application, it is recommended that the
following calculation is used to ensure safe operation at the
C
BOOT_CAP
Q
GATE
V
BOOT_CAP
--------------------------------------
Q
GATE
Q
G1
UVCC
V
GS1
------------------------------------
N
Q1
=
(EQ. 1)
50nC
20nC
FIGURE 2. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE
VOLTAGE
V
BOOT_CAP
(V)
C
BOOT_CAP
(µF)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.30.0 0.1 0.2 0.4 0.5 0.6 0.90.7 0.8 1.0
Q
GATE
= 100nC
ISL6594A, ISL6594B
