LT1776CS8 datasheet(11/20 Pages) LINER

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LT1776

APPLICATIONS INFORMATION

output controls the shutdown pin allows high efficiency at

light loads through Burst Mode operation behavior (see

Typical Applications and Figure 8).

Maximum Load/Short-Circuit Considerations

The LT1776 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 2V, 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 turnoff

of the output switch. These result 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 LT1776 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

when the FB pin voltage is abnormally low thereby indicat-

ing some sort of short-circuit condition. Figure 2 shows

the typical response of Oscillator Frequency vs FB divider

Thevenin voltage and impedance. Oscillator frequency is

unaffected until FB voltage drops to about 2/3 of its normal

value. Below this point the oscillator frequency decreases

roughly linearly down to a limit of about 30kHz. This lower

oscillator frequency during short-circuit conditions can

then maintain control with the effective minimum ON time.

A further potential problem with short-circuit operation

might occur if the user were operating the part with its

oscillator slaved to an external frequency source via the

SYNC pin. However, the LT1776 has circuitry that auto-

matically disables the sync function when the oscillator is

slowed down due to abnormally low FB voltage.

FB DIVIDER THEVENIN VOLTAGE (V)

100

150

200

0.25

0.50

0.75

1.00

1776 F02

1.25

RTH

LT1776

RTH = 22k

RTH = 10k

RTH = 4.7k

Figure 2. Oscillator Frequency vs FB Divider

Thevenin Voltage and Impedance

Feedback Divider Considerations

An LT1776 application typically includes a resistive divider

between VOUT and ground, the center node of which drives

the FB pin to the reference voltage VREF. This establishes

a fixed ratio between the two resistors, but a second

degree of freedom is offered by the overall impedance level

of the resistor pair. The most obvious effect this has is one

of efficiency—a higher resistance feedback divider will

waste less power and offer somewhat higher efficiency,

especially at light load.

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