7

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 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 0.1

µF 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 automatically initializes 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. 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

incorporates specialized circuitry which insures that the

complementary MOSFETs are not ON simultaneously.

The adaptive shoot-through protection utilized by the

ISL6526 looks 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

divider is used to scale the output voltage relative to the

reference voltage and feed it back to the inverting input of

the error amplifier, see Figure 2. However, since the value of

R1 affects the values of the rest of the compensation

components, it is advisable to keep its value less than 5k

Ω.

R4 can be calculated based on the following equation:

If the output voltage desired is 0.8V, simply route the output

back to the FB pin through R1, but do not populate R4.

FIGURE 1. SOFT-START INTERVAL

0V

TIME

T2

T3

T0

CPVOUT (5V)

VCC (3.3V)

(1V/DIV)

T1

R4

R1 0.8V

×

–

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=

ISL6526