8
FN9072.9
December 10, 2015
A falling transition on PWM indicates the turn-off of the upper
MOSFET and the turn-on of the lower MOSFET. A short
propagation delay [t
PDLUGATE
] is encountered before the
upper gate begins to fall [t
FUGATE
]. Again, the adaptive shoot-
through circuitry determines the lower gate delay time,
t
PDHLGATE
. The PHASE voltage is monitored and the lower
gate is allowed to rise after PHASE drops below 0.5V. The
lower gate then rises [t
RLGATE
], turning on the lower
MOSFET.
Three-State PWM Input
A unique feature of the HIP660X drivers is the addition of a
shutdown window to the PWM input. If the PWM signal
enters and remains within the shutdown window for a set
holdoff time, the output drivers are disabled and both
MOSFET gates are pulled and held low. The shutdown state
is removed when the PWM signal moves outside the
shutdown window. Otherwise, the PWM rising and falling
thresholds outlined in the Electrical Specifications determine
when the lower and upper gates are enabled.
Adaptive Shoot-Through Protection
Both drivers incorporate adaptive shoot-through protection
to prevent upper and lower MOSFETs from conducting
simultaneously and shorting the input supply. This is
accomplished by ensuring the falling gate has turned off one
MOSFET before the other is allowed to rise.
During turn-off of the lower MOSFET, the LGATE voltage is
monitored until it reaches a 2.2V threshold, at which time the
UGATE is released to rise. Adaptive shoot-through circuitry
monitors the PHASE voltage during UGATE turn-off. Once
PHASE has dropped below a threshold of 0.5V, the LGATE
is allowed to rise. PHASE continues to be monitored during
the lower gate rise time. If PHASE has not dropped below
0.5V within 250ns, LGATE is taken high to keep the
bootstrap capacitor charged. If the PHASE voltage exceeds
the 0.5V threshold during this period and remains high for
longer than 2s, the LGATE transitions low. Both upper and
lower gates are then held low until the next rising edge of the
PWM signal.
Power-On Reset (POR) Function
During initial start-up, the VCC voltage rise is monitored and
gate drives are held low until a typical VCC rising threshold
of 9.95V is reached. Once the rising VCC threshold is
exceeded, the PWM input signal takes control of the gate
drives. If VCC drops below a typical VCC falling threshold of
7.6V during operation, then both gate drives are again held
low. This condition persists until the VCC voltage exceeds
the VCC rising threshold.
Internal Bootstrap Device
The HIP6601B, HIP6603B, and HIP6604B drivers all feature
an internal bootstrap device. Simply adding an external
capacitor across the BOOT and PHASE pins completes the
bootstrap circuit.
The bootstrap capacitor must have a maximum voltage
rating above VCC + 5V. The bootstrap capacitor can be
chosen from the following equation:
Where Q
GATE
is the amount of gate charge required to fully
charge the gate of the upper MOSFET. The V
BOOT
term is
defined as the allowable droop in the rail of the upper drive.
As an example, suppose a HUF76139 is chosen as the
upper MOSFET. The gate charge, Q
GATE
, from the data
sheet is 65nC for a 10V upper gate drive. We will assume a
200mV droop in drive voltage over the PWM cycle. We find
that a bootstrap capacitance of at least 0.325F is required.
The next larger standard value capacitance is 0.33F.
In applications which require down conversion from +12V or
higher and PVCC is connected to a +12V source, a boot
resistor in series with the boot capacitor is required. The
increased power density of these designs tend to lead to
increased ringing on the BOOT and PHASE nodes, due to
faster switching of larger currents across given circuit
parasitic elements. The addition of the boot resistor allows
for tuning of the circuit until the peak ringing on BOOT is
below 29V from BOOT to GND and 17V from BOOT to VCC.
A boot resistor value of 5 typically meets this criteria.
In some applications, a well tuned boot resistor reduces the
ringing on the BOOT pin, but the PHASE to GND peak
ringing exceeds 17V. A gate resistor placed in the UGATE
trace between the controller and upper MOSFET gate is
recommended to reduce the ringing on the PHASE node by
slowing down the upper MOSFET turn-on. A gate resistor
value between 2 to 10 typically reduces the PHASE to
GND peak ringing below 17V.
Gate Drive Voltage Versatility
The HIP6601B and HIP6603B provide the user total
flexibility in choosing the gate drive voltage. The HIP6601B
lower gate drive is fixed to VCC [+12V], but the upper drive
rail can range from 12V down to 5V depending on what
voltage is applied to PVCC. The HIP6603B ties the upper
and lower drive rails together. Simply applying a voltage from
5V up to 12V on PVCC will set both driver rail voltages.
Power Dissipation
Package power dissipation is mainly a function of the
switching frequency and total gate charge of the selected
MOSFETs. Calculating the power dissipation in the driver for
a desired application is critical to ensuring 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. When designing the driver into an
application, it is recommended that the following calculation
C
BOOT
Q
GATE
V
BOOT
------------------------
(EQ. 1)
HIP6601B, HIP6603B, HIP6604B
