LT1777 datasheet(14/24 Pages) LINER

14 / 24 page

LT1777

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 7 shows

the typical response of oscillator frequency vs FB pin

voltage. Oscillator frequency is normal until FB voltage

drops to about half of its normal value. Below this point the

oscillator frequency decreases linearly down to a limit of

about 25kHz. 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 LT1777 has circuitry to automati-

cally disable the sync function when the oscillator is

slowed down due to abnormally low FB voltage.

APPLICATIONS INFORMATION

tors. Their relatively low ESR in the mid-MHz region can

further attenuate high speed glitches.

Maximum Load/Short-Circuit Considerations

The LT1777 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 2.0V, 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. 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 LT1777 clock frequency

of 100kHz, a VIN of 48V and a (VF + I • R) of say, 0.7V, the

maximum tON to maintain control would be approximately

140ns, an unacceptably short time.

FB DIVIDER THEVENIN VOLTAGE (V)

100

120

0.25

0.50

0.75

1.00

1777 F07

1.25

RTH

LT1777

RTH = 10k

RTH = 4.7k

RTH = 22k

Figure 7. Oscillator Frequency vs FB Divider

Thevenin Voltage and Impedance

Feedback Divider Considerations

An LT1777 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 effi-

ciency, especially at light load.

LT1777 Datasheet(PDF) 14 Page - Linear Technology

LT1777 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL