ISL6208 View Datasheet(PDF) - Intersil
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ISL6208 Datasheet PDF : 13 Pages
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ISL6208, ISL6208B
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“Typical Performance Waveforms” on page 7. NOTE: Intersil
does not recommend Diode Emulation use with rDS(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” table on page 3 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 Equation 1:
CBOOT ≥ Δ---Q--V--G-B--A--O--T--O-E--T-
(EQ. 1)
where QGATE is the amount of gate charge required to fully
charge the gate of the upper MOSFET. The ΔVBOOT 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,
QGATE, 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
QGATE = 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
ΔVBOOT_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 shown in Equation 2:
P
=
fs
w
(
1.5
VU
Q
U
+
VL
Q
L
)
+
IV
C
C
V
CC
(EQ. 2)
where fsw is the switching frequency of the PWM signal. VU
and VL represent the upper and lower gate rail voltage. QU and
QL is the upper and lower gate charge determined by MOSFET
selection and any external capacitance added to the gate pins.
The lVCC VCC product is the quiescent power of the driver and
is typically negligible.
1000
QU =100nC
900 QL = 200nC
800
QU = 50nC
QL = 100nC
QU = 50nC
QL = 50nC
700
600
QU = 20nC
500
QL =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
8
FN9115.5
November 22, 2011
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