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Functional Description
Initialization
The HIP6004B automatically initializes upon receipt of power.
Special sequencing of the input supplies is not necessary. The
Power-On Reset (POR) function continually monitors the input
supply voltages. The POR monitors the bias voltage at the V
CC
pin and the input voltage (V
IN
) on the OCSET pin. The level on
OCSET is equal to V
IN
less a fixed voltage drop (see over-
current protection). The POR function initiates soft start
operation after both input supply voltages exceed their POR
thresholds. For operation with a single +12V power source, V
IN
and V
CC
are equivalent and the +12V power source must
exceed the rising V
CC
threshold before POR initiates operation.
Soft Start
The POR function initiates the soft start sequence. An internal
10
μ
A current source charges an external capacitor (C
SS
) on
the SS pin to 4V. Soft start clamps the error amplifier output
(COMP pin) and reference input (+ terminal of error amp) to the
SS pin voltage. Figure 3 shows the soft start interval with
C
SS
= 0.1
μ
F. Initially the clamp on the error amplifier (COMP
pin) controls the converter’s output voltage. At t
1
in Figure 3,
the SS voltage reaches the valley of the oscillator’s triangle
wave. The oscillator’s triangular waveform is compared to the
ramping error amplifier voltage. This generates PHASE pulses
of increasing width that charge the output capacitor(s). This
interval of increasing pulse width continues to t
2
. With sufficient
output voltage, the clamp on the reference input controls the
output voltage. This is the interval between t
2
and t
3
in Figure 3.
At t
3
the SS voltage exceeds the DACOUT voltage and the
output voltage is in regulation. This method provides a rapid
and controlled output voltage rise. The PGOOD signal toggles
‘high’ when the output voltage (V
SEN
pin) is within
±
5% of
DACOUT. The 2% hysteresis built into the power good
comparators prevents PGOOD oscillation due to nominal
output voltage ripple.
Over-Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFET’s on-resistance,
r
DS(ON)
to monitor the current. This method enhances the
converter’s efficiency and reduces cost by eliminating a
current sensing resistor.
The over-current function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R
OCSET
)
programs the over-current trip level. An internal 200
μ
A current
sink develops a voltage across R
OCSET
that is referenced to
V
IN
. When the voltage across the upper MOSFET (also
referenced to V
IN
) exceeds the voltage across R
OCSET
, the
over-current function initiates a soft-start sequence. The soft-
start function discharges C
SS
with a 10
μ
A current sink and
inhibits PWM operation. The soft-start function recharges
C
SS
, and PWM operation resumes with the error amplifier
clamped to the SS voltage. Should an overload occur while
recharging C
SS
, the soft start function inhibits PWM operation
while fully charging C
SS
to 4V to complete its cycle. Figure 4
shows this operation with an overload condition. Note that the
inductor current increases to over 15A during the C
SS
charging interval and causes an over-current trip. The
converter dissipates very little power with this method. The
measured input power for the conditions of Figure 4 is 2.5W.
The over-current function will trip at a peak inductor current
(I
PEAK)
determined by:
I
x R
DS ON
)
where I
OCSET
is the internal OCSET current source
(200
μ
A typical). The OC trip point varies mainly due to the
MOSFET’s r
DS(ON)
variations. To avoid over-current
tripping in the normal operating load range, find the
R
OCSET
resistor 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
where
I is the output inductor ripple current.
,
0V
0V
0V
TIME (5ms/DIV.)
SOFT-START
(1V/DIV.)
OUTPUT
VOLTAGE
(1V/DIV.)
t
2
t
3
PGOOD
(2V/DIV.)
t
1
FIGURE 3. SOFT START INTERVAL
O
S
0A
0V
TIME (20ms/DIV.)
5A
10A
15A
2V
4V
FIGURE 4. OVER-CURRENT OPERATION
I
PEAK
----------------------------------------------------
=
I
PEAK
I
OUT MAX
(
)
I
(
)
2
+
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HIP6004B