NX2113 datasheet(16/19 Pages) MICROSEMI

16 / 19 page

NX2113/2113A

Rev. 2.0

11/18/05

operates. Before half of the start up time, the Feedback

Under Voltage Lock Out comparator is disabled. After

half of start up time, the Feedback UVLO comparator is

enabled. The FB UVLO threshold is set to be half of

voltage at the positive input of error amplifier. With this

set up, if the output is short before soft start, the

Feedback UVLO comparator can catch it and turn off

the driver. The short circuit operation waveform during

normal operation and during the soft start are shown as

follows.

CH3-FB voltage

0.5V/DIV

CH1-SW voltage

10V/DIV

CH4-load current

10A/DIV

Figure 15 - Operation waveforms during short con-

dition.

CH2-output voltage

1V/DIV

CH4-load current

10A/DIV

Figure 16 - Feedback UVLO with start up at

short.

The Feedback UVLO can provide certain short cir-

cuit protection. However, since feedback does not have

accurate information of current, this protection only pro-

vides certain level of over current protection. MOSFET

should design such that it can survive with high pulse

current for a short period of time.

The value of the capacitor on enable pin to ground

and the resistor value of voltage divider on enable pin

should be big enough to keep enable pin high during

short. Otherwise, once output shorts, the input bus volt-

age drops, the chip is disabled before Feedback UVLO

takes effect, and the system goes into hiccup status.

This phenomena is easy to be found during system

startup, if related resistor and capacitor value is not big

enough.

CH2-Output voltage

1V/DIV

CH4-load current

10A/DIV

Figure 17 -Hiccup with start up at short.

Layout Considerations

The layout is very important when designing high

frequency switching converters. Layout will affect noise

pickup and can cause a good design to perform with

less than expected results.

Start to place the power components, make all the

connection in the top layer with wide, copper filled ar-

eas. The inductor, output capacitor and the MOSFET

should be close to each other as possible. This helps to

reduce the EMI radiated by the power traces due to the

high switching currents through them. Place input ca-

pacitor directly to the drain of the high-side MOSFET, to

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