FAN5308 データシートの表示（PDF） - Fairchild Semiconductor

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FAN5308

800mA High-Efficiency Step-Down DC-DC Converter

Fairchild Semiconductor 

The N-channel transistor is turned off before the inductor

current becomes negative. At this time, the P-channel is

switched on again, starting the next pulse. The converter

continues these pulses until the high threshold (typical

1.6% above nominal value) is reached. A higher output

voltage in PFM mode gives additional headroom for the

voltage drop during a load transient from light to full load.

The voltage overshoot during this load transient is also

minimized due to active regulation during turn on of the

N-channel rectifier switch. The device stays in sleep

mode until the output voltage falls below the low thresh-

old. The FAN5308 enters the PWM mode as soon as the

output voltage can no longer be regulated in PFM with

constant peak current.

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:

VOUT = VIN – ILOAD × (RdsON + RL)

EQ. 2

where:

RdsON = P-channel switch on resistance

ILOAD = Output current

RL = Inductor DC resistance

UVLO and Soft Start

The reference and the circuit remain reset until the VIN

crosses its UVLO threshold.

The FAN5308 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 out-

put voltage overshoot. The soft-start is implemented as a

digital circuit, increasing the switch current in four steps

to the P-channel current limit (1500mA). Typical start-up

time for a 20µF output capacitor and a load current of

800mA is 800µs.

Short-Circuit Protection

The switch peak current is limited cycle-by-cycle to a typ-

ical value of 1500mA. 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 200mA.

Thermal Shutdown

When the die temperature exceeds 150°C, a reset

occurs and remains in effect until the die cools to 130°C.

At that time, the circuit is allowed to restart.

©2005 Fairchild Semiconductor Corporation

FAN5308 Rev. 1.0.2

8

www.fairchildsemi.com

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