8

Functional Description

Initialization

The ISL6406 automatically initializes upon receipt of power.

Special sequencing of the input supplies is not necessary.

The Power-On Reset (POR) function continually monitors the

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 2 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.

Frequency Selection

The ISL6406 offers adjustable frequency from 100kHz to

700kHz by changing external resistor connected at pin RT.

Figure 3 shows the typical RT vs Frequency variation curve.

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 ISL6406,

ISL6426 incorporates specialized circuitry which insures that

the MOSFETs are not ON simultaneously.

The adaptive shoot-through protection utilized by the

ISL6406, ISL6426 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 other 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 4. 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.

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 2. SOFT-START INTERVAL

0V

TIME

t2

t3

t0

CPVOUT (5V)

VCC (3.3V)

(1V/DIV)

t1

FIGURE 3. FREQUENCY vs RT

900

800

700

600

500

400

300

200

100

0

22

26.4

33.0

39

44.5

50

64.5

80

100

130

150

180

200

RT (k

Ω)

R4

R1

() 0.8V

()

()

–

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=

ISL6406, ISL6426