ISL6208_07 Datasheet, PDF (7/10 Pages) Intersil Corporation – High Voltage Synchronous Rectified Buck MOSFET Driver

ISL6208
Diode Emulation
Diode emulation allows for higher converter efficiency under
light-load situations. With diode emulation active, the
ISL6208 will detect the zero current crossing of the output
inductor and turn off LGATE. This ensures that
discontinuous conduction mode (DCM) is achieved. Diode
emulation is asynchronous to the PWM signal. Therefore,
the ISL6208 will respond to the FCCM input immediately
after it changes state. Refer to the waveforms on page 6.
NOTE: Intersil does not recommend Diode Emulation use with
r_DS(ON) current sensing topologies. The turn-OFF of the low side
MOSFET can cause gross current measurement inaccuracies.
Three-State PWM Input
A unique feature of the ISL6208 and other Intersil 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 turn on.
During turn-off of the lower MOSFET, the LGATE voltage is
monitored until it reaches a 1V threshold, at which time the
UGATE is released to rise. Adaptive shoot-through circuitry
monitors the upper MOSFET gate-to-source voltage during
UGATE turn-off. Once the upper MOSFET gate-to-source
voltage has dropped below a threshold of 1V, the LGATE is
allowed to rise.
Internal Bootstrap Diode
This driver features an internal bootstrap Schottky diode.
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 the maximum battery voltage plus 5V. The bootstrap
capacitor can be chosen from the following equation:
C_BO
O
T
≥
---^Q----^G-----^A----^T---^E----
ΔV_BOOT
(EQ. 1)
where Q_GATE is the amount of gate charge required to fully
charge the gate of the upper MOSFET. The ΔV_BOOT term is
defined as the allowable droop in the rail of the upper drive.
As an example, suppose an upper MOSFET has a gate
charge, Q_GATE, of 25nC at 5V and also assume the droop in
the drive voltage over a PWM cycle is 200mV. One will find
that a bootstrap capacitance of at least 0.125μF is required.
The next larger standard value capacitance is 0.15μF. A
good quality ceramic capacitor is recommended.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
Q_GATE = 100nC
0.4
0.2 _20nC
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
ΔV_{BOOT_CAP} (V)
FIGURE 8. BOOTSTRAP CAPACITANCE vs BOOT RIPPLE
VOLTAGE
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 SO-8 package is
approximately 800mW. When designing the driver into an
application, it is recommended that the following calculation
be performed to ensure safe operation at the desired
frequency for the selected MOSFETs. The power dissipated
by the driver is approximated as:
P
=
f_sw
(
1.5
V_U
Q
U
+
V_L
Q
L
)
+
I_V
C
C
V
CC
(EQ. 2)
where f_sw is the switching frequency of the PWM signal. V_U
and V_L represent the upper and lower gate rail voltage. Q_U
and Q_L is the upper and lower gate charge determined by
MOSFET selection and any external capacitance added to
the gate pins. The lV_CC V_CC product is the quiescent power
of the driver and is typically negligible.
1000
Q_U =100nC
900 Q_L = 200nC
800
Q_U = 50nC
Q_L = 100nC
Q_U = 50nC
Q_L = 50nC
700
600
Q_U = 20nC
500
Q_L =50nC
400
300
200
100
0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
FREQUENCY (kHz)
FIGURE 9. POWER DISSIPATION vs FREQUENCY
7
FN9115.2
March 30, 2007