AP2003

Synchronous PWM Controller

(Preliminary)

Anachip Corp.

www.anachip.com.tw

Rev. 0.1 Feb.10, 2004

5/6

Function Description

Synchronous Buck Converter

voltage-mode pulse width modulated (PWM)

controller. This section has all the features required

to build a high efficiency synchronous buck

converter, shutdown function.

The output voltage of the synchronous converter is

set and controlled by the output of the error

amplifier. The external resistive divider reference

voltage is derived from an internal trimmed

band-gap voltage reference. The inverting input of

the error amplifier receives its voltage from the

SENSE pin.

The internal oscillator uses an on-chip capacitor

and trimmed precision current sources to set the

oscillation frequency to 200KHz. The triangular

output of the oscillator sets the reference voltage at

the inverting input of the comparator. When the

oscillator output voltage drops below the error

amplifier output voltage, the comparator output

goes high. This pulls DRVL low, turning off the

low-side FET, and DRVH is pulled high, turning on

the high-side FET (once the cross-current control

allows it). When the oscillator voltage rises back

above the error amplifier output voltage, the

comparator output goes low. This pulls DRVH low,

turning off the high-side FET, and DRVL is pulled

high, turning on the low-side FET (once the

cross-current control allows it).

As SENSE increases, the output voltage of the

error amplifier decreases. This causes a reduction

in the on-time of the high-side MOSFET connected

to DRVH, hence lowering the output voltage.

Under Voltage Lockout

The under voltage lockout circuit of the AP2003

assures that the high-side MOSFET driver outputs

remain in the off state whenever the supply voltage

falls below 4.1V. Normal operation resumes once

Soft Start

charge an external capacitor. The outputs of the

error

amplifiers

are

clamped

to

a

voltage

limits the on-time of the high-side MOSFETs, thus

leading to a controlled ramp-up of the output

voltages.

Hiccup Mode

10uA of current to the external soft-start capacitor.

As the soft-start voltage rises, the output of the

internal error amplifier is clamped to this voltage.

When the error signal reaches the level of the

internal triangular oscillator, which swings from 1V

to 2V at a fixed frequency of 200KHz, switching

occurs. As the error signal crosses over the

oscillator signal, the duty cycle of the PWM signal

continues to increase until the output comes into

regulation.

The soft-start voltage will begin to decrease as the

2uA of current discharge the external capacitor.

When the soft-start voltage reaches 0.8V, the

to charge the external capacitor causing the

soft-start voltage to rise again. Again, when the

soft-start voltage reaches the level of the internal

oscillator, switching will occur.

In conclusion, above is shown a typical “12V

Application Circuit” which has a BSTH voltage

derived by bootstrapping input voltage to the

PHASE node through diode D2. This circuit is very

useful in cases where only single input power of

5V(or 12V) is available.

In

order

to

prevent

substrate

glitching,

a

small-signal diode should be placed in close

proximity to the chip with cathode connected to

PHASE and anode connected to GND.