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APW7077

PWM Step-Up DC-DC Converter

Anpec Electronics 

APW7077/A

Function Description (Cont.)

Operation (Cont.)

inductor current is higher than the output current, then

sustains the output voltage until the next switching

cycle.

As the load current is decreased, the switch transistor

turns on for a shorter duty cycle. Under the light load

condition, the controller will skip switching cycles to

reduce power consumption, so that high efficiency is

maintained at light loads.

Fixed Output Voltage (for APW7077 only)

The APW7077 VOUT is set by an integrate feedback

resistor network. This is trimmed to a selected voltage

3.3 V or 5.0 V with an accuracy of +/-2.5%.

Setting Output Voltage (for APW7077A only)

For APW7077A, the output voltage is adjustable. The

output voltage is set using the FB pin and a resistor

divider connected to the output as shown in the typical

operating circuit. The internal reference voltage is 1.0V

with 2% variation, so the ratio of the feedback resistors

sets the output voltage according to the following

equation:

R2

VOUT = (1 + ) × 1.0V

R1

To avoid the thermal noise from feedback resistor,

(R1+R2) resistance smaller than 1MΩ and 1% variation

is recommended.

Soft Start

There is a soft start function is integration in

APW7077/A series to avoid the over shooting when

power on. When power is applied to the device, the

soft start circuit first pumps up the output voltage to

let VDD(or VOUT) approximately 1.65V at a fixed duty

cycle 50%. This is the voltage level at which the

controller can operate normally. When supply voltage

more than 1.65V the internal reference voltage will be

ramp up to let output voltage reach to setting voltage

without over shooting issue whenever heavy load or

light load condition. The soft start time 25ms is

setting by internal circuit.

Oscillator

The oscillator frequency is internally set to 300 kHz at

an accuracy of +/-10% and with low temperature

coefficient of 3.3%/°C.

Enable/Disable Operation

The APW7077/Aseries offer IC shutdown mode by chip

enable pin (CE pin) to reduce current consumption.

When voltage at pin CE is greater than 1.2 V, the

chip will be enabled, which means the controller is

in normal operation. When voltage at pin CE is less

than 0.7 V, the chip is disabled, which means IC is

shutdown and quiescent current become 1uA.

The CE pin pull high to VDD(or VOUT) by internal resistor,

and this resistance is greater than 1MΩ . So this chip

will enable normally when CE pin floating.

Important: DO NOT apply a voltage between 0.7V

to 1.2 V to pin CE as this is the CE pin’s hysteresis

voltage range. Clearly defined output states can

only be obtained by applying voltage out of this

range.

Compensation

The device is designed to operate in continuous

conduction mode. An internal compensation circuit

was designed to guarantee stability over the full

input/output voltage and full output load range.

Step–up Converter Operating Mode

The step–up DC–DC controller is designed to operate

in continuous conduction mode (CCM) or discontinuous

conduction mode (DCM).

For a step up converter in a CCM, the duty cycle D is

Copyright © ANPEC Electronics Corp.

12

Rev. A.4 - Sep, 2005

www.anpec.com.tw

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