LM3421 datasheet(28/72 Pages) TI1

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)

fSW =

25 x VIN - VO

RT x CT X VIN

(

)

VIN

Boost Inrush Diode

LM3421, LM3421-Q1, LM3423, LM3423-Q1

SNVS574F – JULY 2008 – REVISED JULY 2015

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Application Information (continued)

8.1.7 Boost Inrush Current

When configured as a boost converter, there is a phantom power path comprised of the inductor, the output

diode, and the output capacitor. This path will cause two things to happen when power is applied. First, there will

be a very large inrush of current to charge the output capacitor. Second, the energy stored in the inductor during

this inrush will end up in the output capacitor, charging it to a higher potential than the input voltage.

Depending on the state of the EN pin, the output capacitor would be discharged by:

1. EN < 1.3 V: no discharge path (leakage only).

2. EN > 1.3 V, the OVP divider resistor path, if present, and 10 µA into each of the HSP & HSN pins.

In applications using the OVP divider and with EN > 1.3 V, the output capacitor voltage can charge higher than

VTURN-OFF. In this situation, the FLT pin (LM3423 only) is open and the PWM dimming MOSFET is turned off.

This condition (the system appearing disabled) can persist for an undesirably long time. Possible solutions to this

condition are:

•

Add an inrush diode from VIN to the output as shown in Figure 30.

•

Add an NTC thermistor in series with the input to prevent the inrush from overcharging the output capacitor

too high.

•

Use a current limited source supply.

•

Raise the OVP threshold.

Figure 30. Boost Topology with Inrush Diode

8.1.8 Switching Frequency

An external resistor (RT) connected between the RCT pin and the switch node (where D1, Q1, and L1 connect),

in combination with a capacitor (CT) between the RCT and AGND pins, sets the off-time (tOFF) as shown in

Figure 31. For boost and buck-boost topologies, the VIN proportionality ensures a virtually constant switching

frequency (fSW).

For a buck topology, RT and CT are also used to set tOFF, however the VIN proportionality will not ensure a

constant switching frequency. Instead, constant ripple operation can be achieved. Changing the connection of RT

in Figure 31 from VSW to VIN will provide a constant ripple over varying VIN. Adding a PNP transistor as shown in

Figure 32 will provide constant ripple over varying VO.

The switching frequency is defined:

Buck (Constant Ripple vs. VIN)

(31)

Buck (Constant Ripple vs. VO)

(32)

Boost and Buck-boost

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LM3421 Datasheet(PDF) 28 Page - Texas Instruments

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