ISL6526AEVAL2 datasheet(8/18 Pages) RENESAS

8 / 18 page

ISL6526, ISL6526A

FN9055 Rev 12.00

Page 8 of 18

Sep 30, 2015

OCSET

Connect a resistor (ROCSET) from this pin to the drain of the

upper MOSFET (VIN). ROCSET, an internal 20µA current

source (IOCSET), and the upper MOSFET ON-resistance

(rDS(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

VIN and the internal reference, 0.8V. An external resistor

divider is used to scale the output voltage relative to the

IPEAK

IOCSETxROCSET

rDS ON



-------------------------------------------------

(EQ. 1)

FIGURE 1. SOFT-START INTERVAL

TIME

CPVOUT (5V)

VCC (3.3V)

VOUT (2.50V)

(1V/DIV)

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