6

LT1372/LT1377

The LT1372/LT1377 are current mode switchers. This

means that switch duty cycle is directly controlled by

switch current rather than by output voltage. Referring to

the block diagram, the switch is turned “On” at the start of

each oscillator cycle. It is turned “Off” when switch current

reaches a predetermined level. Control of output voltage is

obtained by using the output of a voltage sensing error

amplifier to set current trip level. This technique has

several advantages. First, it has immediate response to

input voltage variations, unlike voltage mode switchers

which have notoriously poor line transient response.

Second, it reduces the 90

° phase shift at mid-frequencies

in the energy storage inductor. This greatly simplifies

closed-loop frequency compensation under widely vary-

ing input voltage or output load conditions. Finally, it

allows simple pulse-by-pulse current limiting to provide

maximum switch protection under output overload or

short conditions. A low dropout internal regulator pro-

vides a 2.3V supply for all internal circuitry. This low

dropout design allows input voltage to vary from 2.7V to

25V with virtually no change in device performance. A

500kHz (LT1372) or 1MHz (LT1377) oscillator is the basic

clock for all internal timing. It turns “On” the output switch

via the logic and driver circuitry. Special adaptive anti-sat

circuitry detects onset of saturation in the power switch

and adjusts driver current instantaneously to limit switch

saturation. This minimizes driver dissipation and provides

very rapid turn-off of the switch.

A 1.245V bandgap reference biases the positive input of

the error amplifier. The negative input of the amplifier is

brought out for positive output voltage sensing. The error

amplifier has nonlinear transconductance to reduce out-

put overshoot on start-up or overload recovery. When

the feedback voltage exceeds the reference by 40mV,

error amplifier transconductance increases ten times,

which reduces output overshoot. The feedback input also

invokes oscillator frequency shifting, which helps pro-

tect components during overload conditions. When the

feedback voltage drops below 0.6V, the oscillator fre-

quency is reduced 5:1. Lower switching frequency allows

full control of switch current limit by reducing minimum

switch duty cycle.

Unique error amplifier circuitry allows the LT1372/LT1377

to directly regulate negative output voltages. The negative

feedback amplifier’s 100k source resistor is brought out

for negative output voltage sensing. The NFB pin regulates

at – 2.49V while the amplifier output internally drives the

FB pin to 1.245V. This architecture, which uses the same

main error amplifier, prevents duplicating functions and

maintains ease of use. Consult Linear Technology Market-

ing for units that can regulate down to – 1.25V.

The error signal developed at the amplifier output is

functions. It is used for frequency compensation, current

limit adjustment and soft starting. During normal regula-

tor operation this pin sits at a voltage between 1V (low

output current) and 1.9V (high output current). The error

be externally clamped for lowering current limit. Like-

wise, a capacitor coupled external clamp will provide soft

below the control pin threshold, placing the LT1372/

LT1377 in an idle mode.

APPLICATIO S I FOR ATIO

Positive Output Voltage Setting

from the FB pin to ground. Output voltage is set by

connecting the FB pin to an output resistor divider

(Figure 1). The FB pin bias current represents a small

error and can usually be ignored for values of R2 up to 7k.

The suggested value for R2 is 6.19k. The NFB pin is

normally left open for positive output applications.

Figure 1. Positive Output Resistor Divider

R1

R2

FB

PIN

R2

R1 = R2

– 1

()

1.245

LT1372 • F01