ISL6522B Datasheet, PDF (7/15 Pages) Intersil Corporation – Buck and Synchronous Rectifier Pulse-Width Modulator (PWM) Controller

ISL6522B
Functional Description
Initialization
The ISL6522B 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.
During Soft-Start, the ISL6522B controls the regulator in a
standard buck fashion. The lower MOSFET is not enabled
during soft-start. The body diode of the MOSFET or the
external diode, if used, will conduct when the upper
MOSFET is OFF. Once the output has reached regulation,
the lower MOSFET is enabled and the regulator is controlled
as a synchronous buck regulator. This allows the ISL6522B
regulator to start into a pre-biased output.
During Soft Start, the ISL6522B functions as a standard buck
converter by disabling the lower MOSFET. This is done by
holding the LGATE pin LOW. If there is not a diode in parallel
with the lower MOSFET, the body diode of the lower
MOSFET will conduct when the upper MOSFET is off. Once
the SS pin has reached it’s peak value, the lower MOSFET
is enabled and the ISL6522B functions as a synchronous
buck converter.
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
7