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LT1432

Figure 17 shows the effect of a 500mA transient load (0.3A

to 0.8A) on the LT1432, both with and without an output

filter. The top trace with no filter shows about a 60mV

deviation with a settling time of 300

µs. Astute switching

regulator designers may notice the lack of switching ripple

in this trace. To make a clean display the actual trace was

fed through a one pole filter with 16

µs time constant to

eliminate most of the switching ripple. This had very little

effect on the shape or amplitude of the response waveform

(you’ll have to trust me on this one). In the middle trace,

an output filter of 10

µH and 200µF was added to the

regulator to achieve very low output ripple. The load

transient response is obviously degraded because the

second filter capacitor, following normal design practice,

is somewhat smaller than the main output capacitor, and

therefore also has higher ESR. Note the slight ringing

caused by the “Q” of the output filter. Calculated ringing

frequency is 1/(2

√LC) = 3.4kHz. Also note the small step

in DC level between the two load conditions on the filtered

output. To maintain good loop stability, the added filter is

left “outside” the feedback loop. Therefore, the DC resis-

tance of the 10

µH inductor will add to load regulation. The

10mV step implies a resistance of 10mV/0.5A = 0.02

Ω.

The message in all this is to be careful when adding output

filters if transient load response or load regulation is

critical. The second filter capacitor may have to be as large

as the main filter capacitor.

APPLICATI

I FOR ATIO

Mode Pin Drive

The mode pin defines operating conditions for the LT1432.

A low state programs the IC to operate in “normal” mode

as a constant frequency, current mode, buck converter.

Floating the pin converts the internal error amplifier to a

comparator which puts the LT1432 into a low-power

“burst” mode. In this mode, the pin assumes an open

circuit voltage of approximately 1V. To ensure stable

operation, current into or out of the pin must be limited to

µA. If the pin is routed near any switching or logic signals

it should be bypassed with a 200pF capacitor to avoid

pickup.

Driving the mode pin high causes the LT1432 to go into

complete shutdown. An internal resistor limits mode pin

current to about 15

µA at 5V. A 7V zener diode is also in

parallel with the pin, so input voltages higher than 6.5V

must be externally limited with a resistor. The current/

voltage characteristics of the mode pin are shown in

Typical Performance Characteristics. Note that the drive

signal must sink about 30

µA when pulling the mode pin to

its worst case low threshold of 0.6V. This should not be a

problem for any standard open drain or three-state output.

If all three states are desired and a three-state drive is not

available, the circuit shown in Figure 18 can be used. Two

separate logic inputs are used. Both low will allow the

mode pin to float for burst mode. “A” high, “B” low will

generate shutdown, and “B” high, “A” low forces normal

mode operation. Both high will also force normal mode

operation, but this is not an intended state and R1 is

included to limit overload of “A” if this occurs. C1 is

suggested if the mode pin line can pick up capacitively

coupled stray switching or logic signals.

100mV/DIV

0.5A/DIV

0.5ms/DIV

MODE PIN

200pF

VN2222L

10k

1N914

LT1432 F18

Figure 18. Two Input Mode Drive

Figure 17

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