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FAN2011/FAN2012 Rev. 1.0.2

Block Diagram

Figure 4. Block Diagram

Detailed Operation Description

The FAN2011 is a step-down pulse width modulated (PWM)

current mode converter with a typical switching frequency of 1.3

MHz. At the beginning of each clock cycle, the P-channel tran-

sistor is turned on. The inductor current ramps up and is moni-

tored via an internal circuit. The P-channel switch is turned off

when the sensed current causes the PWM comparator to trip

when the output voltage is in regulation or when the inductor

current reaches the current limit (set internally to typically

2600mA). 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 solely given by the ratio

of output voltage and input voltage. Therefore, the converter

runs with a minimum duty cycle when output voltage is at mini-

mum and input voltage is at maximum.

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-chan-

nel transistor continuously on. In this mode the output voltage is

equal to the input voltage minus the voltage drop across the P-

channel transistor:

UVLO and Soft Start

its UVLO threshold.

The FAN2011 has an internal soft-start circuit that limits the in-

rush 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 cur-

rent limit (2600mA). Typical start-up time for a 40µF output

capacitor and a load current of 1500mA is 800µs.

Short Circuit Protection

The switch peak current is limited cycle by cycle to a typical

value of 2600mA. In the event of an output voltage short circuit,

the device operates with a frequency of 400kHz and minimum

duty cycle, therefore the average input current is typically

350mA.

Thermal Shutdown

When the die temperature exceeds 150°C, a reset occurs and

will remain in effect until the die cools to 130°C, at that time the

circuit will be allowed to restart.

Applications Information

Setting the Output Voltage

The internal reference is 0.8V. The output is divided down by a

voltage divider, R1 and R2 to the FB pin. The output voltage is:

According to this equation, and assuming desired output volt-

age of 1.5096V, and given R2 = 10K

Ω, the calculated value of

R1 is 8.87K

Ω.

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

EN

GND

0.8V

V

OUT

V

R

1

R

2

-------

+





=