ISL6525_04 Datasheet, PDF (5/11 Pages) Intersil Corporation – Buck and Synchronous-Rectifier Pulse-Width Modulator (PWM) Controller

ISL6525
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_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 VCC threshold before POR initiates operation.
Soft Start
The POR function initiates the soft start sequence. An internal
10µA 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 = 0.1µF. Initially the clamp on the error amplifier (COMP
pin) controls the converter’s output voltage. At t₁ 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 t₂. With
sufficient output voltage, the clamp on the reference input
controls the output voltage. This is the interval between t₂ and
t₃ in Figure 3. At t₃ the SS voltage exceeds the reference
voltage and the output voltage is in regulation. This method
provides a rapid and controlled output voltage rise.
Over-Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFETs 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 200µA (typical)
current sink develops a voltage across R_OCSET that is
reference 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 10µA 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, in this particular
application, 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 average input power for the conditions of Figure 4 is
2.5W.
4V
2V
0V
15A
10A
5A
0A
SOFT-START
(1V/DIV.)
OUTPUT
VOLTAGE
0V
(1V/DIV.)
0V
t₁
t₂
t₃
TIME (5ms/DIV.)
FIGURE 3. SOFT-START INTERVAL
TIME (20ms/DIV.)
FIGURE 4. OVER-CURRENT OPERATION
The over-current function will trip at a peak inductor current
(I_PEAK) determined by:
I_PEAK
=
I_{--O-----C----S----E----T-----}•_----R_{----O-----C----S----E----T--}
r_DS(ON)
where I_OCSET is the internal OCSET current source (200µA
- typical). The OC trip point varies mainly due to the
MOSFETs 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.
5
FN4998.3
December 27, 2004