FAC1591 datasheet(11/12 Pages) FS

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Over Current Protection (OCP)

The cycle by cycle current limit threshold is set

between 3A and 4A. When the load current

reaches the current limit threshold, the cycle by

cycle current limit circuit turns off the high side

switch immediately to terminate the current duty

cycle. The inductor current stops rising. The cycle

by cycle current limit protection directly limits

inductor peak current. The average inductor

current is also limited due to the limitation on peak

inductor current. When the cycle by cycle current

limit circuit is triggered, the output voltage drops

as the duty cycle is decreasing.

Thermal Management and Layout

Consideration

In the FAC1591 buck regulator circuit, high pulsing

current flows through two circuit loops. The first

loop starts from the input capacitors, to the VIN

pin, to the VOUT pins, to the filter inductor, to the

output capacitor and load,and then returns to the

input capacitor through ground.

Current flows in the first loop when the high side

switch is on. The second loop starts from the

inductor, to the output capacitors and load, to the

GND pin of the FAC1591, and to the VOUT pins of

the FAC1591. Current flows in the second loop

when the low side diode is on.

In PCB layout, minimizing the two loops area

reduces the noise of this circuit and improves

efficiency. A ground plane is recommended to

connect input capacitor,

output capacitor, and

GND pin of the FAC1591.

In the FAC1591 buck regulator circuit, the two

major power dissipating components are the

FAC1591 and output inductor. The total power

dissipation of convertercircuit can be measured

by input power minus output power.

Ptotal _loss

= V IN × IIN – V O × IO

The power dissipation of inductor can be

approximately calculated by output current and

DCR of inductor.

Pinductor _loss

= IO 2 × Rinductor × 1.1

The junction to ambient temperature can be got

from power dissipation in the FAC1591 and thermal

impedance from junction to ambient.

T (jun-amb)

=(Ptotalloss–Pinductorloss)× ΘJA

The maximum junction temperature of FAC1591 is

145°C, which limits the maximum load current

capability. Please see the thermal de-rating

curves for the maximum load current of the

FAC1591 under different ambient temperatures.

The thermal performance of the FAC1591 is trongly

affected by the PCB layout. Extra care should be

taken by users during the design process to nsure

that the IC will operate under the recommended

environmental conditions.

Several layout tips are listed below for the best

electric and thermal performance.

1. Do not use thermal relief connection to the VIN

and the GND pin. Pour a maximized copper area

to the GND pin and the VIN pin to help thermal

dissipation.

2. Input capacitor should be connected to the VIN

pin and the GND pin as close as possible.

3. Make the current trace from VOUT pins to L to

the GND as short as possible.

4. Pour copper plane on all unused board area

and connect it to stable DC nodes, like VIN, GND,

or VOUT.

5. Keep sensitive signal traces such as trace

connecting FB pin away from the VOUT pins.

2008.01. 28

11/12

Revision No : 0

FAC1591

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