ISL6522A Datasheet, PDF (6/13 Pages) Intersil Corporation – Buck and Synchronous Rectifier Pulse-Width Modulator (PWM) Controller

ISL6522A
monitored by the adaptive shoot through protection circuitry to
determine when the lower MOSFET has turned off.
PVCC
Provide a bias supply for the lower gate drive to this pin.
VCC
Provide a 12V bias supply for the chip to this pin.
Functional Description
Initialization
The ISL6522A 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 and the enable (EN) pin.
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 overcurrent
protection). With the EN pin held to V_CC, 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.
The POR function inhibits operation with the chip disabled
(EN pin low). With both input supplies above their POR
thresholds, transitioning the EN pin high initiates a soft-start
interval.
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) to the SS pin voltage. Figure 3 shows the
soft-start interval. At t₁ in Figure 3, the SS and COMP
voltages reach 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, at which
point the output is in regulation and the clamp on the COMP
pin is released. This method provides a rapid and controlled
output voltage rise.
6
VOLTAGE
V_{SOFT START}
V_OUT
V_OSC(MIN)
V_COMP
CLAMP ON V_COMP
RELEASED AT STEADY STATE
t₀
t₁
t₂
TIME
t₁
=
C_{-----S----S--}
I_SS
⋅
V_O
S
C
(
M
IN)
t_SoftStart
=
t₂ – t₁
=
-^C----^S----^S--
I_SS
⋅
-^V----^O----^U----^T----^S----^t--^e---^a---^d----^y---^S----^t-^a----^t--^e-
V_IN
⋅
∆
V_OS
C
Where:
C_SS = Soft Start Capacitor
I_SS = Soft Start Current = 10µA
V_OSC(MIN) = Bottom of Oscillator = 1.35V
V_IN = Input Voltage
∆V_OSC = Peak to Peak Oscillator Voltage = 1.9V
V_{OUTSteadyState} = Steady State Output Voltage
FIGURE 3. SOFT-START INTERVAL
4V
2V
0V
15A
10A
5A
0A
TIME (20ms/DIV)
FIGURE 4. OVERCURRENT OPERATION
Overcurrent Protection
The overcurrent 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 overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R_OCSET)
programs the overcurrent trip level. An internal 200µA
(typical) current sink develops a voltage across R_OCSET that
FN9122.2
April 13, 2005