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FAN5331 Rev. 1.0.1

Block Diagram

Figure 4. Block Diagram

Circuit Description

The FAN5331 is a pulse-width modulated (PWM) current-mode

boost converter. The FAN5331 improves the performance of bat-

tery powered equipment by significantly minimizing the spectral

distribution of noise at the input caused by the switching action of

the regulator. In order to facilitate effective noise filtering, the

switching frequency was chosen to be high, 1.6MHz. An internal

soft start circuitry minimizes in-rush currents. The timing of the soft

start circuit was chosen to reach 95% of the nominal output voltage

The device architecture is that of a current mode controller with

an internal sense resistor connected in series with the N-chan-

nel switch. The voltage at the feedback pin tracks the output

voltage at the cathode of the external Schottky diode (shown in

the test circuit). The error amplifier amplifies the difference

between the feedback voltage and the internal bandgap refer-

ence. The amplified error voltage serves as a reference voltage

to the PWM comparator. The inverting input of the PWM com-

parator consists of the sum of two components: the amplified

control signal received from the 50m

Ω current sense resistor

and the ramp generator voltage derived from the oscillator. The

oscillator sets the latch, and the latch turns on the FET switch.

Under normal operating conditions, the PWM comparator resets

the latch and turns off the FET, thus terminating the pulse.

Since the comparator input contains information about the out-

put voltage and the control loop is arranged to form a negative

feedback loop, the value of the peak inductor current will be

adjusted to maintain regulation.

Every time the latch is reset, the FET is turned off and the cur-

rent flow through the switch is terminated. The latch can be

reset by other events as well. Over-current condition is moni-

tored by the current limit comparator which resets the latch and

turns off the switch instantaneously within each clock cycle.

Over-Voltage Protection

The voltage on the feedback pin is sensed by an OVP Compar-

ator. When the feedback voltage is 15% higher than the nominal

voltage, the OVP Comparator stops switching of the power tran-

sistor, thus preventing the output voltage from going higher.

Applications Information

Setting the Output Voltage

The internal reference is 1.23V (Typical). The output voltage is

divided by a resistor divider, R1 and R2 to the FB pin. The out-

put voltage is given by

According to this equation, and assuming desired output volt-

age of 15V, good choices for the feedback resistors are,

Inductor Selection

The inductor parameters directly related to device performances

are saturation current and dc resistance. The FAN5331 oper-

ates with a typical inductor value of 10µH. The lower the dc

resistance, the higher the efficiency. Usually a trade-off between

inductor size, cost and overall efficiency is needed to make the

optimum choice.

Reference

Oscillator

n

FB

FB

Driver

Over

Voltage

Comp

Comp

-

-

+

+

S

Ramp

Generator

R

R

R

Q

S

Current Limit

Comparator

0.05

Shutdown

Circuitry

Thermal

Shutdown

SHDN

GND

SW

Amp

Error

Amp

45

1

2

3

+

+

-

-

+

-

Soft-Start

V

OUT

V

R

1

R

2

-------

+

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