8
FN9055.12
September 30, 2015
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OCSET
Connect a resistor (R
OCSET
) from this pin to the drain of the
upper MOSFET (V
IN
). 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 Equation 1:
An overcurrent trip cycles the soft-start function.
ENABLE
This pin is the open-collector enable pin. Pulling this pin to a
level below 0.8V will disable the controller. Disabling the
ISL6526, ISL6526A causes the oscillator to stop, the LGATE
and UGATE outputs to be held low, and the soft-start
circuitry to re-arm.
CT1 and CT2
These pins are the connections for the external charge
pump capacitor. A minimum of a 0.1µF ceramic capacitor is
recommended for proper operation of the IC.
CPVOUT
This pin represents the output of the charge pump. The
voltage at this pin is the bias voltage for the IC. Connect a
decoupling capacitor from this pin to ground. The value of the
decoupling capacitor should be at least 10x the value of the
charge pump capacitor. This pin may be tied to the bootstrap
circuit as the source for creating the BOOT voltage.
CPGND
This pin represents the signal and power ground for the
charge pump. Tie this pin to the ground island/plane through
the lowest impedance connection available.
Functional Description
Initialization
The ISL6526, ISL6526A automatically initialize upon receipt
of power. Special sequencing of the input supplies is not
necessary. The Power-On Reset (POR) function continually
monitors the output voltage of the charge pump. During
POR, the charge pump operates on a free running oscillator.
Once the POR level is reached, the charge pump oscillator
is synched to the PWM oscillator. The POR function also
initiates the soft-start operation after the charge pump output
voltage exceeds its POR threshold.
Soft-Start
The POR function initiates the digital soft-start sequence.
The PWM error amplifier reference is clamped to a level
proportional to the soft-start voltage. As the soft-start voltage
slews up, the PWM comparator generates PHASE pulses of
increasing width that charge the output capacitor(s). This
method provides a rapid and controlled output voltage rise.
The soft-start sequence typically takes about 6.5ms.
Figure 1 shows the soft-start sequence for a typical application.
At t0, the +3.3V VCC voltage starts to ramp-up. At time t1, the
Charge Pump begins operation and the +5V CPVOUT IC bias
voltage starts to ramp-up. Once the voltage on CPVOUT
crosses the POR threshold at time t2, the output begins the
soft-start sequence. The triangle waveform from the PWM
oscillator is compared to the rising error amplifier output
voltage. As the error amplifier voltage increases, the pulse
width on the UGATE pin increases to reach the steady-state
duty cycle at time t3.
Shoot-Through Protection
A shoot-through condition occurs when both the upper
MOSFET and lower MOSFET are turned on simultaneously,
effectively shorting the input voltage to ground. To protect
the regulator from a shoot-through condition, the ISL6526,
ISL6526A incorporate specialized circuitry which insures
that the complementary MOSFETs are not ON
simultaneously.
The adaptive shoot-through protection utilized by the
ISL6526, ISL6526A look at the lower gate drive pin, LGATE,
and the upper gate drive pin, UGATE, to determine whether
a MOSFET is ON or OFF. If the voltage from UGATE or from
LGATE to GND is less than 0.8V, then the respective
MOSFET is defined as being OFF and the complementary
MOSFET is turned ON. This method of shoot-through
protection allows the regulator to sink or source current.
Since the voltage of the lower MOSFET gate and the upper
MOSFET gate are being measured to determine the state of
the MOSFET, the designer is encouraged to consider the
repercussions of introducing external components between
the gate drivers and their respective MOSFET gates before
actually implementing such measures. Doing so may
interfere with the shoot-through protection.
Output Voltage Selection
The output voltage can be programmed to any level between
V
IN
and the internal reference, 0.8V. An external resistor
divider is used to scale the output voltage relative to the
I
PEAK
I
OCSET
xR
OCSET
r
DS ON
-------------------------------------------------=
(EQ. 1)
FIGURE 1. SOFT-START INTERVAL
0V
TIME
t2
t3
t0
CPVOUT (5V)
VCC (3.3V)
V
OUT
(2.50V)
(1V/DIV)
t1
ISL6526, ISL6526A
