10

LT1676

Power loss internal to the LT1676 related to actual output

current is composed of both DC and AC switching losses.

These can be roughly estimated as follows:

DC switching losses are dominated by output switch “ON

voltage”, i.e.,

DC = ON duty cycle

AC switching losses are typically dominated by power lost

Assuming, for simplicity, a linear ramp up of both voltage

and current and a current rise/fall time equal to 15ns,

f = switching frequency

Total power dissipation of the die is simply the sum of

quiescent, DC and AC losses previously calculated.

Frequency Compensation

Loop frequency compensation is performed by connect-

ing a capacitor, or in most cases a series RC, from the

loop compensation may be obtained by empirical meth-

ods as described in detail in Application Note 19. Briefly,

this involves applying a load transient and observing the

As a practical matter, a second small capacitor, directly

value for this capacitor is 100pF. (See Switch Node Con-

siderations).

Switch Node Considerations

For maximum efficiency, switch rise and fall times are

made as short as practical. To prevent radiation and high

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 LT1676 has circuitry that auto-

matically disables the sync function when the oscillator is

slowed down due to abnormally low FB voltage.

Feedback Divider Considerations

An LT1676 application typically includes a resistive divider

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.

However, remember that oscillator slowdown to achieve

short-circuit protection (discussed above) is dependent

on FB pin behavior, and this in turn, is sensitive to FB node

external impedance. Figure 2 shows the typical relation-

ship between FB divider Thevenin voltage and impedance,

and oscillator frequency. This shows that as feedback

network impedance increases beyond 10k, complete os-

cillator slowdown is not achieved, and short-circuit pro-

tection may be compromised. And as a practical matter,

the product of FB pin bias current and larger FB network

impedances will cause increasing output voltage error.

(Nominal cancellation for 10k of FB Thevenin impedance

is included internally.)

Thermal Considerations

Care should be taken to ensure that the worst-case input

voltage and load current conditions do not cause exces-

sive die temperatures. The packages are rated at 110

°C/W

for the 8-pin SO (S8) and 130

°C/W for 8-pin PDIP (N8).

Quiescent power is given by: