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over-voltage on the PWM output disables all converters and
drives the FAULT pin to VCC.
Figure 7 shows a simplified schematic of the fault logic. An
over-voltage detected on VSEN1 immediately sets the fault
latch. A sequence of three over-current fault signals also
sets the fault latch. A comparator indicates when C
SS
is fully
charged (UP signal), such that an under-voltage event on
either linear output (FB2 or FB3) is ignored until after the
soft-start interval (T4 in Figure 6). At startup, this allows
V
OUT3
to slew up without generating a fault. Cycling the bias
input voltage (+12V
IN
on the VCC pin) off then on resets the
counter and the fault latch. Shutting down VOUT1 and
VOUT3 also resets the counter and latch.
Over-Voltage Protection
During operation, a short on the upper PWM MOSFET (Q1)
causes V
OUT1
to increase. When the output exceeds the
over-voltage threshold of 115% (typical) of DACOUT, the
over-voltage comparator trips to set the fault latch and turns
Q2 on as required in order to regulate V
OUT1
to 1.15 x
DACOUT. This blows the input fuse and reduces V
OUT1
.
The fault latch raises the FAULT pin close to VCC potential.
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), V
OUT1
is
monitored for voltages exceeding 1.26V. Should VSEN1
exceed this level, the lower MOSFET (Q2) is driven on as
needed to regulate V
OUT1
to 1.26V.
Over-Current Protection
All outputs are protected against excessive over-currents.
The PWM controller uses the upper MOSFET’s on-
resistance, r
DS(ON)
to monitor the current for protection
against shorted outputs. The linear regulator restricts the
current through the integrated power device to 230mA
(typically). The linear regulator and the linear controller
monitor VOUT2 and VSEN3, respectively, for under-voltage
to protect against excessive currents.
Figures 8 and 9 illustrate the over-current protection with an
overload on V
OUT1
. The overload is applied at T0 and the
current increases through the output inductor (L
OUT1
). At
time T1, the OVER-CURRENT1 comparator trips when the
voltage across Q1 (I
D
r
DS(ON)
) exceeds the level
programmed by R
OCSET
. This inhibits all outputs,
discharges the soft-start capacitor (C
SS
) with an 11
μ
A
current sink, and increments the counter. C
SS
recharges at
T2 and initiates a soft-start cycle with the error amplifiers
clamped by soft-start. With OUT1 still overloaded, the
inductor current increases to trip the over-current
comparator. Again, this inhibits all outputs, but the soft-start
voltage continues increasing to 4V before discharging. The
counter increments to 2. The soft-start cycle repeats at T3
and trips the over-current comparator. The SS pin voltage
increases to 4V at T4 and the counter increments to 3. This
sets the fault latch to disable the converter. The fault is
reported on the FAULT pin.
The linear regulator and linear controller monitor the outputs
for an under-voltage. Should excessive currents cause
VOUT2 or VSEN3 to fall below the linear under-voltage
threshold, the LUV signal sets the over-current latch if C
SS
is
fully charged. Blanking the LUV signal during the C
SS
charge
interval allows the linear outputs to build above the under-
voltage threshold during normal start-up. Cycling the bias
input power off then on resets the counter and the fault latch.
Resistor R
OCSET
programs the over-current trip level for the
PWM converter. As shown in Figure 9, the internal 200
μ
A
current sink develops a voltage across R
OCSET
(V
SET
) that
is referenced to V
IN
. The DRIVE signal enables the over-
current comparator (OVER-CURRENT1). When the voltage
across the upper MOSFET (V
DS(ON)
) exceeds V
SET
, the
over-current comparator trips to set the over-current 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
FAULT
LATCH
S
R
Q
POR
SD1&3
COUNTER
OC1
OV
LUV
+
-
+
-
0.15V
4V
SS
VCC
FAULT
R
FIGURE 7. FAULT LOGIC - SIMPLIFIED SCHEMATIC
UP
OVER
CURRENT
LATCH
INHIBIT
S
R
Q
S
0A
0V
2V
4V
FIGURE 8. OVER-CURRENT OPERATION
TIME
T1
T2
T3
T0
T4
F
0V
10V
OVERLOAD
APPLIED
C= 1
C= 2
C= 3
FAULT
REPORTED
HIP6028