ISL6523A Datasheet, PDF (8/16 Pages) Intersil Corporation – VRM8.5 Dual PWM and Dual Linear Power System Controller

ISL6523A
and the soft-start signals (SS13, SS24). Window
comparators monitor the SS pins and indicate when the
respective C_SS pins are fully charged to above 4.0V (UP
signals). An under-voltage on either linear output (VSEN3 or
VSEN4) is ignored until the respective UP signal goes high.
This allows V_OUT3 and V_OUT4 to increase without fault at
start-up. Following an overcurrent event (OC1, OC2, or UV3
event), bringing the SS24 pin below 0.8V resets the
overcurrent latch and generates a soft-started ramp-up of
the outputs 1, 2, and 3.
OUT1 Overvoltage Protection
During operation, a short across the synchronous PWM
upper MOSFET (Q1) causes V_OUT1 to increase. When the
output exceeds the over-voltage threshold of 120% of
DACOUT, the over-voltage comparator trips to set the fault
latch and turns the lower MOSFET (Q2) on as needed to
regulate the output voltage to the 120% threshold. This
operation typically results in the blow of the input fuse,
subsequent discharge of V_OUT1.
A separate over-voltage circuit provides protection during
the initial application of power. For voltages on the VCC pin
below the power-on reset (and above ~4V), the output level
is monitored for voltages above 1.3V. Should VSEN1 exceed
this level, the lower MOSFET, Q2, is driven on.
Overcurrent Protection
All outputs are protected against excessive overcurrents.
Both PWM controllers use the upper MOSFET’s on-
resistance, r_DS(ON) to monitor the current for protection
against shorted outputs. Both linear regulators monitor their
respective VSEN pins for under-voltage to protect against
excessive currents.
Figure 7 illustrates the overcurrent protection with an overload
on OUT2. The overload is applied at T0 and the current
increases through the inductor (L_OUT2). At time T1, the OC2
comparator trips when the voltage across Q3 (i_D • r_DS(ON))
exceeds the level programmed by R_OCSET. This inhibits
outputs 1, 2, and 3, discharges soft-start capacitor C_SS24 with
28µA current sink, and increments the counter. Soft-start
capacitor C_SS13 is quickly discharged. C_SS24 recharges at T2
and initiates a soft-start cycle with the error amplifiers clamped
by soft-start. With OUT2 still overloaded, the inductor current
increases to trip the overcurrent comparator. Again, this
inhibits the outputs, but the soft-start voltage continues
increasing to above 4.0V before discharging. The counter
increments to 2. The soft-start cycle repeats at T3 and trips
the overcurrent comparator. The SS pin voltage increases to
above 4.0V at T4 and the counter increments to 3. This sets
the fault latch to disable the converter.
The PWM1 controller operates in the same way as PWM2 to
overcurrent faults. Additionally, the two linear controllers
monitor the VSEN pins for under-voltage. Should excessive
currents cause VSEN3 or VSEN4 to fall below the linear
under-voltage threshold, the respective UV signals set the
OC latch or the FAULT latch, providing respective C_SS
capacitors are fully charged. Blanking the UV signals during the
C_SS charge interval allows the linear outputs to build above
the under-voltage threshold during normal operation. Cycling
the bias input power off then on resets the counter and the
fault latch.
CHIP
1
DISABLED
0
COUNT
=1
4V
COUNT
=2
2V
0V
OVERLOAD
APPLIED
COUNT
=3
0A
T0 T1
T2
T3
T4
TIME
FIGURE 7. OVERCURRENT OPERATION
Resistors (R_OCSET1 and R_OCSET2) program the overcurrent
trip levels for each PWM converter. As shown in Figure 8, the
internal 200µA current sink (I_OCSET) develops a voltage across
R_OCSET (V_SET) that is referenced to V_IN. The DRIVE signal
enables the overcurrent comparator (OVERCURRENT1 or
OVERCURRENT2). When the voltage across the upper
MOSFET (V_DS(ON)) exceeds V_SET, the overcurrent
comparator trips to set the overcurrent latch. Both V_SET and
V_DS are referenced to V_IN and a small capacitor across
R_OCSET helps V_OCSET track the variations of V_IN due to
MOSFET switching. The overcurrent function will trip at a peak
inductor current (I_PEAK) determined by:
I_PEAK = -^I-^O-----^C----^S----^E_r--_D--^T--_S---^×-₍---_O^R----_N-^O---₎-^C-----^S---^E----^T--
The OC trip point varies with MOSFET’s rDS(ON)
temperature variations. To avoid overcurrent tripping in the
normal operating load range, determine the ROCSET
resistor value from the equation above with:
1. The maximum r_DS(ON) at the highest junction temperature
2. The minimum I_OCSET from the specification table
3. Determine I_PEAK for I_PEAK > I_OUT(MAX) + (∆I)/ 2,
where ∆I is the output inductor ripple current.
For an equation for the ripple current see the section under
component guidelines titled ‘Output Inductor Selection’.
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