isl6236 Intersil Corporation, isl6236 Datasheet - Page 28

• Manufacturer Part Number: isl6236
• Description: High-efficiency, Quad-output, Main Power Supply Controllers For Notebook Computers
• Manufacturer: Intersil Corporation

100mV when ILIM is connected to VCC. The logic threshold
for switch-over to the 100mV default value is approximately
VCC -1V.
The PC board layout guidelines should be carefully
observed to ensure that noise and DC errors do not corrupt
the current-sense signals at PHASE.
MOSFET Gate Drivers (UGATE, LGATE )
The UGATE and LGATE gate drivers sink 2.0A and 3.3A
respectively of gate drive, ensuring robust gate drive for
high-current applications. The UGATE floating high-side
MOSFET drivers are powered by diode-capacitor charge
pumps at BOOT. The LGATE synchronous-rectifier drivers
are powered by PVCC.
The internal pull-down transistors that drive LGATE low have
a 0.6Ω typical on-resistance. These low on-resistance
pulldown transistors prevent LGATE from being pulled up
during the fast rise time of the inductor nodes due to
capacitive coupling from the drain to the gate of the low-side
synchronous-rectifier MOSFETs. However, for high-current
applications, some combinations of high- and low-side
MOSFETs may cause excessive gate-drain coupling, which
leads to poor efficiency and EMI-producing shoot-through
currents. Adding a 1Ω resistor in series with BOOT
increases the turn-on time of the high-side MOSFETs at the
expense of efficiency, without degrading the turn-off time
(Figure 75).
Adaptive dead-time circuits monitor the LGATE and UGATE
drivers and prevent either FET from turning on until the other
is fully off. This algorithm allows operation without shoot-
through with a wide range of MOSFETs, minimizing delays
and maintaining efficiency. There must be low-resistance,
low-inductance paths from the gate drivers to the MOSFET
gates for the adaptive dead-time circuit to work properly.
Otherwise, the sense circuitry interprets the MOSFET gate
as "off" when there is actually charge left on the gate. Use
very short, wide traces measuring 10 to 20 squares (50 mils
to 100 mils wide if the MOSFET is 1” from the device).
FIGURE 75. REDUCING THE SWITCHING-NODE RISE TIME
ISL88732
ISL88732
ISL88733
ISL88733
ISL88734
ISL88734
ISL6236
ISL6236
5V
5V
5V
5V
UGATE
UGATE
UGATE
UGATE
BOOT
BOOT
BOOT
BOOT
BOOT
C BOOT
10Ω
10
10
10
10
PHASE
28
Q1
VIN
OUT
OUT
OUT
OUT
ISL6236
Boost-Supply Capacitor Selection (Buck)
The boost capacitor should be 0.1µF to 4.7µF, depending on
the input and output voltages, external components, and PC
board layout. The boost capacitance should be as large as
possible to prevent it from charging to excessive voltage, but
small enough to adequately charge during the minimum
low-side MOSFET conduction time, which happens at
maximum operating duty cycle (this occurs at minimum input
voltage). The minimum gate to source voltage (V
determined by:
where:
• PVCC is 5V
• C
Boost-Supply Refresh Monitor
In pure skip mode, the converter frequency can be very low
with little to no output loading. This produces very long off
times, where leakage can bleed down the BOOT capacitor
voltage. If the voltage falls too low, the converter may not be
able to turn on UGATE when the output voltage falls to the
reference. To prevent this, the ISL6236 monitors the BOOT
capacitor voltage, and if it falls below 3V, it initiates an
LGATE pulse, which will refresh the BOOT voltage.
POR, UVLO, and Internal Digital Soft-Start
Power-on reset (POR) occurs when VIN rises above
approximately 3V, resetting the undervoltage, overvoltage,
and thermal-shutdown fault latches. PVCC
undervoltage-lockout (UVLO) circuitry inhibits switching
when PVCC is below 4V. LGATE is low during UVLO. The
output voltages begin to ramp up once PVCC exceeds its 4V
UVLO and REF is in regulation. The internal digital soft-start
timer begins to ramp up the maximum-allowed current limit
during start-up. The 1.7ms ramp occurs in five steps. The
step size are 20%, 40%, 60%, 80% and 100% of the positive
current limit value.
Power-Good Output (POK )
The POK comparator continuously monitors both output
voltages for undervoltage conditions. POK is actively held
low in shutdown, standby, and soft-start. POK1 releases and
digital soft-start terminates when VOUT1 outputs reach the
error-comparator threshold. POK1 goes low if VOUT1 output
turns off or is 10% below its nominal regulation point. POK1
is a true open-drain output. Likewise, POK2 is used to
monitor VOUT2.
Fault Protection
The ISL6236 provides overvoltage/undervoltage fault
protection in the buck controllers. Once activated, the
V
GS MIN
GS
(
is the gate capacitance of the high-side MOSFET
)
=
PVCC
⋅
-------------------------------------- -
C
BOOT
C
BOOT
+
C
GS
GS(MIN)
June 28, 2007
FN6373.3
(EQ. 5)
) is