Functional Pin Description
This is the main bias supply for the ISL6520, as well as the
lower MOSFET’s gate. Connect a well-decoupled 5V supply
to this pin.
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.
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.
Connect this pin to the upper MOSFET source. This pin is
used to monitor the voltage drop across the upper MOSFET
for over-current protection. This pin is also monitored by the
continuously adaptive shoot-through protection circuitry to
determine when the upper MOSFET has turned off.
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. Do not insert any circuitry between
this pin and the gate of the upper MOSFET, as it may
interfere with the internal adaptive shoot-through protection
circuitry and render it ineffective.
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.
This is a multiplexed pin. During a short period of time following
power-on reset (POR), this pin is used to determine the over-
current threshold of the converter. Connect a resistor
(ROCSET) from this pin to the drain of the upper MOSFET
(VCC). ROCSET, an internal 20µA current source (IOCSET), and
the upper MOSFET on-resistance (rDS(ON)) set the converter
over-current (OC) trip point according to the following
IPEAK = I--O-----C----S---r-E-D---T-S---x-(--RO-----ON----)C----S----E----T--
Internal circuitry of the ISL6520 will not recognize a voltage
drop across ROCSET larger than 0.5V. Any voltage drop
across ROCSET that is greater than 0.5V will set the
overcurrent trip point to:
IPEAK = r---D---0-S--.--5(--O-V---N-----)
An over-current 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 FB pin, to compensate the
voltage-control feedback loop of the converter.
Pulling OCSET to a level below 0.8V will disable the
controller. Disabling the ISL6520 causes the oscillator to
stop, the LGATE and UGATE outputs to be held low, and the
softstart circuitry to re-arm.
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.
Do not insert any circuitry between this pin and the gate of the
lower MOSFET, as it may interfere with the internal adaptive
shoot-through protection circuitry and render it ineffective.
The ISL6520 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
Over-Current 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.
Over Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFET’s on-resistance,
rDS(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
(ROCSET) programs the over-current trip level (see Typical
Immediately following POR, the ISL6520 initiates the Over-
Current Protection sampling and hold operation. First, the
internal error amplifier is disabled. This allows an internal
20µA current sink to develop a voltage across ROCSET. The
ISL6520 then samples this voltage at the COMP pin. This
sampled voltage, which is referenced to the VCC pin, is held
internally as the Over-Current Set Point.
When the voltage across the upper MOSFET, which is also
referenced to the VCC pin, exceeds the Over-Current Set
Point, the over-current 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
ISL6520 to go into a hiccup mode with a typical period of
25ms. The inductor current increases to 18A during the Soft