© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN2013 Rev. 1.0.3

6

Block Diagram

FB

ERROR

AMP

REF FB

IS

CURRENT

SENSE

DIGITAL

SOFT START

UNDER VOLTAGE

LOCKOUT

LOGIC

CONTROL

MOSFET

DRIVER

OSC

SLOPE COMPENSATION

COMP

OVER

VOLTAGE

COMP

IS

GND

SW

PG

GND

0.8V

PG

COMP

GND

REF

FB

Figure 10.

Block Diagram

Detailed Operation Description

The FAN2013 is a step-down pulse-width modulated

(PWM) current mode converter with a fixed switching

frequency of 1.3MHz. At the rising edge of each clock

cycle, the P-channel transistor is turned on until the

PWM comparator trips or the current limit is reached.

During the ON time, the inductor current ramps up and

is monitored by the internal current-mode control loop.

After a minimum dead time, the N-channel transistor is

turned ON and the inductor current ramps down. As the

clock cycle is completed, the N-channel switch is turned

OFF and the next clock cycle starts. The duty cycle is

given by the ratio of output voltage and input voltage.

The converter runs at minimum duty cycle when output

voltage is at minimum and input voltage is at maximum,

and at 100% duty cycle when the input voltage

approaches the output voltage, as described below.

100% Duty Cycle Operation

As the input voltage approaches the output voltage and

the duty cycle exceeds the typical 95%, the converter

turns the P-channel transistor continuously on. In this

mode, the output voltage is equal to the input voltage,

minus the voltage drop across the P-channel transistor:

(1)

where

UVLO and Soft Start

The FAN2013 has an internal soft-start circuit that limits

the inrush current during start-up. This prevents

possible voltage drops of the input voltage and

eliminates the output voltage overshoot. The soft-start

is implemented as a digital circuit, increasing the switch

current in four steps to the P-channel current limit

(3.5A). Typical start-up time for a 40µF output capacitor

with a load current of 2.0A is 800µs.

Output Over-Voltage Protection

above the nominal value, the device turns OFF the P-

channel switch and turns ON part of the N-channel

transistor with a built-in current limit of about 400mA.

device starts switching normally in closed loop. If output

voltage is pulled up by an external voltage source with a

current limit higher than typical 400mA, the output

voltage stays up at the external voltage source level.

The over-voltage protection is designed to limit the

output voltage excursion in case of a transient response

from full load to a minimum load.

Output Short-Circuit Protection

The switch peak current is limited cycle by cycle to a

typical value of 3.5A. In the event of an output voltage

short circuit, the device operates with a frequency of

400kHz and minimum duty cycle, making the average

typical input current .45A.

Thermal Shutdown

When the die temperature exceeds 150°C, a reset

occurs and remains in effect until the die cools to

130°C, when the circuit is allowed to restart.