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LT3437

3437fc

VOUT

1V/DIV

VIN = 12V

1ms/DIV

3437 F04

COUT = 100µF

ILOAD = 200mA

CSS = GND

CSS = 0.1µF

CSS = 0.01µF

Figure 4. VOUT dV/dt

Short-Circuit Considerations

The LT3437 is a current mode controller. It uses the VC

node voltage as an input to a current comparator which

turns off the output switch on a cycle-by-cycle basis as

this peak current is reached. The internal clamp on the VC

node, nominally 1.5V, then acts as an output switch peak

current limit. This action becomes the switch current limit

specification. The maximum available output power is

then determined by the switch current limit.

A potential controllability problem could occur under

short-circuit conditions. If the power supply output is

short circuited, the feedback amplifier responds to the low

output voltage by raising the control voltage, VC, to its

peak current limit value. Ideally, the output switch would

be turned on, and then turned off as its current exceeded

the value indicated by VC. However, there is finite response

time involved in both the current comparator and turn-off

of the output switch. This results in a minimum on time

tON(MIN). When combined with the large ratio of VIN to

(VF + I • R), the diode forward voltage plus inductor I • R

voltage drop, the potential exists for a loss of control.

Expressed mathematically the requirement to maintain

control is:

VI R

•

≤

where:

f = switching frequency

tON = switch on time

VF = diode forward voltage

VIN = Input voltage

I • R = inductor I • R voltage drop

If this condition is not observed, the current will not be

limited at IPK but will cycle-by-cycle ratchet up to some

higher value. Using the nominal LT3437 clock frequency

of 200kHz, a VIN of 40V and a (VF + I • R) of say 0.7V, the

maximum tON to maintain control would be approximately

90ns, an unacceptably short time.

The solution to this dilemma is to slow down the oscillator

to allow the current in the inductor to drop to a sufficiently

low value such that the current does not continue to

ratchet higher. When the FB pin voltage is abnormally low,

thereby indicating some sort of short-circuit condition,

the oscillator frequency will be reduced. Oscillator fre-

quency is reduced by a factor of 10 when the FB pin voltage

is below 0.4V and increases linearly to its typical value of

200kHz at a FB voltage of 0.95V (see Typical Performance

Characteristics). These oscillator frequency reductions

during short-circuit conditions allow the LT3437 to main-

tain current control

SOFT-START

For applications where [VIN/(VOUT + VF)] >10 or large input

surge currents cannot be tolerated, the LT3437 soft-start

feature should be used to control the output capacitor

charge rate during start-up, or during recovery from an

output short circuit, thereby adding additional control

over peak inductor current. The soft-start function limits

the switch current via the VC pin to maintain a constant

voltage ramp rate (dV/dt) at the output capacitor. A capaci-

tor (C1 in Figure 2) from the CSS pin to the regulated output

voltage determines the output voltage ramp rate. When

the current through the CSS capacitor exceeds the CSS

threshold (ICSS), the voltage ramp of the output capacitor

is limited by reducing the VC pin voltage.The CSS threshold

is proportional to the FB voltage (see Typical Performance

Characteristics) and is defeated for FB voltages greater

than 0.9V (typical). The output dV/dt can be approxi-

mated by:

CSS

but actual values will vary due to start-up load conditions,

compensation values and output capacitor selection.

APPLICATIO S I FOR ATIO

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