ISL6236A Datasheet, PDF (30/37 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6236A
where:
• PVCC is 5V
• C_GS is the gate capacitance of the high-side MOSFET
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 ISL6236A 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 V_IN 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 ISL6236A provides overvoltage/undervoltage fault
protection in the buck controllers. Once activated, the
controller continuously monitors the output for undervoltage
and overvoltage fault conditions.
OUT-OF-BOUND CONDITION
When the output voltage is 5% above the set voltage, the
out-of-bound condition activates. LGATE turns on until
output reaches within regulation. Once the output is within
regulation, the controller will operate as normal. It is the "first
line of defense" before OVP. The output voltage ripple must
be sized low enough as to not nuisance trip the OOB
threshold. The equations in “Output Capacitor Selection” on
page 33 should be used to size the output voltage ripple
below 3% of the nominal output voltage set point.
OVERVOLTAGE PROTECTION
When the output voltage of VOUT1 is 11% (16% for VOUT2)
above the set voltage, the overvoltage fault protection
activates. This latches on the synchronous rectifier MOSFET
with 100% duty cycle, rapidly discharging the output
capacitor until the negative current limit is achieved. Once
negative current limit is met, UGATE is turned on for a
minimum on-time, followed by another LGATE pulse until
negative current limit. This effectively regulates the
discharge current at the negative current limit in an effort to
prevent excessively large negative currents that cause
potentially damaging negative voltages on the load. Once an
overvoltage fault condition is set, it can only be reset by
toggling SHDN, EN, or cycling VIN (POR).
UNDERVOLTAGE PROTECTION
When the output voltage drops below 70% of its regulation
voltage for at least 100µs, the controller sets the fault latch
and begins the discharge mode (see “Shutdown Mode” and
“Discharge Mode” on page 30). UVP is ignored for at least
20ms (typical), after start-up or after a rising edge on EN.
Toggle EN or cycle VIN (POR) to clear the undervoltage fault
latch and restart the controller. UVP only applies to the buck
outputs.
THERMAL PROTECTION
The ISL6236A has thermal shutdown to protect the devices
from overheating. Thermal shutdown occurs when the die
temperature exceeds +150°C. All internal circuitry shuts
down during thermal shutdown. The ISL6236A may trigger
thermal shutdown if LDO is not bootstrapped from OUT
while applying a high input voltage on VIN and drawing the
maximum current (including short circuit) from LDO. Even if
LDO is bootstrapped from OUT, overloading the LDO causes
large power dissipation on the bootstrap switches, which
may result in thermal shutdown. Cycling EN, EN LDO, or
VIN (POR) ends the thermal-shutdown state.
Discharge Mode (Soft-Stop)
When a transition to standby or shutdown mode occurs, or
the output undervoltage fault latch is set, the outputs
discharge to GND through an internal 25Ω switch. The
reference remains active to provide an accurate threshold
and to provide overvoltage protection.
Shutdown Mode
The ISL6236A SMPS1, SMPS2 and LDO have independent
enabling control. Drive EN1, EN2 and EN LDO below the
precise input falling-edge trip level to place the ISL6236A in
its low-power shutdown state. The ISL6236A consumes only
20µA of quiescent current while in shutdown. When
shutdown mode activates, the 3.3V VREF3 remain on. Both
SMPS outputs are discharged to 0V through a 25Ω switch.
30
FN6453.3
March 18, 2008