7

FN9056.10

November 18, 2008

Functional Description

Initialization

The ISL6527, ISL6527A automatically initialize 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

is 1.5V

proportionally smaller as shown in Equation 2:

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 ISL6527,

ISL6527A incorporate specialized circuitry, which insures that

the complementary MOSFETs are not ON simultaneously.

The adaptive shoot-through protection utilized by the

ISL6527, ISL6527A 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.

PROTECTION/DISABLE

OCSET Current Source

Commercial

182022

µA

Industrial

16

20

22

µA

Disable Threshold

--

0.8

V

NOTE:

4. Limits should be considered typical and are not production tested.

Electrical Specifications

PARAMETER

SYMBOL

TEST CONDITIONS

MIN

TYP

MAX

UNITS

RiseTime

t

SS

V

REFIN

1.5V

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

×

=

(EQ. 2)

FIGURE 1. SOFT-START INTERVAL

0V

TIME

t2

t3

t0

VCC (3.3V)

VOUT (2.50V)

(1V/DIV)

t1

CPVOUT (5V)

ISL6527, ISL6527A