ISL6520A_07 Datasheet, PDF (5/12 Pages) Intersil Corporation – Single Synchronous Buck Pulse-Width Modulation (PWM) Controller

ISL6520A
Functional Pin Description
VCC
This pin provides the bias supply for the ISL6520A, as well
as the lower MOSFET’s gate. Connect a well-decoupled 5V
supply to this pin.
FB
This pin is the inverting input of the internal error amplifier.
Use this pin, in combination with the COMP/OCSET pin, to
compensate the voltage-control feedback loop of the
converter.
GND
This pin represents the signal and power ground for the IC.
Tie this pin to the ground island/plane through the lowest
impedance connection available.
PHASE
Connect this pin to the upper MOSFET’s source. This pin is
used to monitor the voltage drop across the upper MOSFET
for overcurrent protection.
UGATE
Connect this pin to the upper MOSFET’s gate. This pin
provides the PWM-controlled gate drive for the upper
MOSFET. This pin is also monitored by the adaptive shoot-
through protection circuitry to determine when the upper
MOSFET has turned off.
BOOT
This pin provides ground referenced bias voltage to the
upper MOSFET driver. A bootstrap circuit is used to create a
voltage suitable to drive a logic-level N-channel MOSFET.
COMP/OCSET
This is a multiplexed pin. During a short period of time
following power-on reset (POR), this pin is used to determine
the overcurrent threshold of the converter. Connect a
resistor (R_OCSET) from this pin to the drain of the upper
MOSFET (V_CC). R_OCSET, an internal 20μA current source
(I_OCSET), and the upper MOSFET on-resistance (r_DS(ON))
set the converter overcurrent (OC) trip point according to the
following equation:
I_PEAK
=
I--_{O-----C----S----E----T----x---R-----O----C----S----E----T--}
r_DS(ON)
(EQ. 1)
Internal circuitry of the ISL6520A will not recognize a voltage
drop across R_OCSET larger than 0.5V. Any voltage drop
across R_OCSET that is greater than 0.5V will set the
overcurrent trip point to:
I_PEAK
=
------0---.--5---V--------
r_DS(ON)
(EQ. 2)
An overcurrent trip cycles the soft-start function.
During soft-start, and all the time during normal converter
operation, this pin represents the output of the error
amplifier. Use this pin, in combination with the
COMP/OCSET pin, to compensate the voltage-control
feedback loop of the converter.
Pulling COMP/OCSET to a level below 0.8V disables the
controller. Disabling the ISL6520A causes the oscillator to
stop, the LGATE and UGATE outputs to be held low, and the
softstart circuitry to re-arm.
LGATE
Connect this pin to the lower MOSFET’s gate. This pin
provides the PWM-controlled gate drive for the lower
MOSFET. This pin is also monitored by the adaptive shoot-
through protection circuitry to determine when the lower
MOSFET has turned off.
Functional Description
Initialization
The ISL6520A automatically initializes upon receipt of power.
The Power-On Reset (POR) function continually monitors the
bias voltage at the VCC pin. The POR function initiates the
Overcurrent Protection (OCP) sampling and hold operation
after the supply voltage exceeds its POR threshold. Upon
completion of the OCP sampling and hold operation, the POR
function initiates the soft-start operation.
Overcurrent Protection
The overcurrent 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 overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R_OCSET)
programs the overcurrent trip level (see Typical Application
diagram).
Immediately following POR, the ISL6520A initiates the
Overcurrent Protection sampling and hold operation. First,
the internal error amplifier is disabled. This allows an internal
20mA current sink to develop a voltage across R_OCSET. The
ISL6520A then samples this voltage at the COMP pin. This
sampled voltage, which is referenced to the VCC pin, is held
internally as the Overcurrent Set Point.
When the voltage across the upper MOSFET, which is also
referenced to the VCC pin, exceeds the Overcurrent Set
Point, the overcurrent function initiates a soft-start sequence.
Figure 1 shows the inductor current after a fault is introduced
while running at 15A. The continuous fault causes the
ISL6520A to go into a hiccup mode with a typical period of
25ms. The inductor current increases to 18A during the soft-
start interval and causes an overcurrent trip. The converter
dissipates very little power with this method. The measured
input power for the conditions of Figure 1 is only 1.5W.
5
FN9016.5
March 28, 2007