ISL6525 Intersil Corporation, ISL6525 Datasheet - Page 5

• Manufacturer Part Number: ISL6525
• Description: Buck and Synchronous-Rectifier Pulse-Width Modulator (PWM) Controller
• Manufacturer: Intersil Corporation

PGOOD signal, not the falling edge. An internal hysteresis
guarantees glitch-free transition of PGOOD. Refer to
Programming PGOOD Delay Time section for more
information.
Functional Description
Initialization
The ISL6525 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 VCC
pin and the input voltage (V
OCSET is equal to V
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
and V
exceed the rising VCC threshold before POR initiates operation.
Soft Start
The POR function initiates the soft start sequence. An internal
10 A current source charges an external capacitor (C
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
pin) controls the converter’s output voltage. At t
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
sufficient output voltage, the clamp on the reference input
controls the output voltage. This is the interval between t
t
voltage and the output voltage is in regulation. This method
provides a rapid and controlled output voltage rise.
3
SS
0V
0V
in Figure 3. At t
= 0.1 F. Initially the clamp on the error amplifier (COMP
CC
are equivalent and the +12V power source must
FIGURE 3. SOFT-START INTERVAL
SOFT-START
(1V/DIV.)
3
the SS voltage exceeds the reference
IN
less a fixed voltage drop (see over-
t
1
TIME (5ms/DIV.)
IN
) on the OCSET pin. The level on
5
t
2
OUTPUT
VOLTAGE
(1V/DIV.)
t
3
1
in Figure 3,
2
. With
SS
2
) on
and
IN
ISL6525
Over-Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFETs on-resistance,
r
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
programs the over-current trip level. An internal 200 A (typical)
current sink develops a voltage across R
reference to V
(also referenced to V
the over-current function initiates a soft-start sequence. The
soft-start function discharges C
inhibits PWM operation. The soft-start function recharges C
and PWM operation resumes with the error amplifier clamped
to the SS voltage. Should an overload occur while recharging
C
charging C
operation with an overload condition. Note that, in this particular
application, the inductor current increases to over 15A during
the C
converter dissipates very little power with this method. The
measured average input power for the conditions of Figure 4 is
2.5W.
The over-current function will trip at a peak inductor current
(I
where I
- typical). The OC trip point varies mainly due to the
MOSFETs r
in the normal operating load range, find the R
from the equation above with:
I
DS(ON)
1. The maximum r
PEAK
PEAK)
SS
, the soft start function inhibits PWM operation while fully
15A
10A
4V
2V
5A
0A
SS
0V
=
OCSET
determined by:
charging interval and causes an over-current trip. The
to monitor the current. This method enhances the
I
-------------------------------------------------- -
OCSET
SS
FIGURE 4. OVER-CURRENT OPERATION
DS(ON)
IN
to 4V to complete its cycle. Figure 4 shows this
r
DS ON
is the internal OCSET current source (200 A
. When the voltage across the upper MOSFET
R
DS(ON)
variations. To avoid over-current tripping
IN
OCSET
) exceeds the voltage across R
TIME (20ms/DIV.)
at the highest junction temperature.
SS
with a 10 A current sink and
OCSET
OCSET
that is
OCSET
OCSET
resistor
SS
)
,
,