6

LT1373

The LT1373 is a current mode switcher. 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

°phaseshiftatmid-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

250kHz 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 LT1373 to

directly regulate negative output voltages. The negative

feedback amplifier’s 400k source resistor is brought out

for negative output voltage sensing. The NFB pin regulates

at – 2.45V 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 Mar-

keting 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 LT1373 in an

idle mode.

Positive Output Voltage Setting

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 25k. The suggested value for

R2 is 24.9k. The NFB pin is normally left open for positive

output applications.

R1

R2

FB

PIN

R2

R1 = R2

– 1

()

1.245

LT1373 • F01

Figure 1. Positive Output Resistor Divider

OPERATIO

APPLICATIO S I FOR ATIO