LT3513

12

3513fa

All four switchers employ a constant-frequency current

mode control scheme. Switcher 1, the step-down regula-

tor, differs slightly from the others with inductor current

sense. Instead of monitoring the current at the switch,

current nodes are used to measure the current through

the inductor. Inductor current sense does not suffer from

minimum on-time problems, therefore always keep-

ing the switch current limited with any input-to-output

voltage ratio. Switcher 1 is always synchronized to the

master oscillator. The other three switchers each have

their own slave oscillator. The slave oscillator reduces the

frequency when the feedback voltage dips below 0.75V

and decreases linearly below the threshold as shown in

the Performance Characteristics’ Frequency Foldback plot.

Other than these two differences, the control loop is similar

in all four switchers. A pulse from the master oscillator

for switcher 1 or a pulse from the slave oscillator for the

other three switchers sets the RS latch and turns on the

internal NPN bipolar power switch. Current in the switch

and the external inductor begins to increase. When this

current comparator resets the latch, turning off the switch.

The current in the inductor flows through the Schottky

diode and begins to decrease. The cycle begins again at

the next pulse from the oscillator. In this way, the voltage

the output. The internal error amplifier regulates the output

or duty cycle. Switcher 1’s current limit is controlled by

ers also use slope compensation to ensure stability with

the current mode scheme at duty cycles above 50%. The

RUN-SS1, RUN-SS2 and RUN-SS3/4 pins control the rate

of rise of the feedback pins.

the BOOST pin. An external capacitor and an integrated

Schottky diode are used to generate a voltage at the BOOST

pin that is higher than the input supply. This allows the

driver to saturate the internal bipolar NPN power switch

for efficient operation.

INPUT VOLTAGE RANGE STEP-DOWN CONSIDERATION

The minimum operating voltage of switcher 1 is determined

either by the LT3513’s undervoltage lockout of ~4V or by

its maximum duty cycle. A user defined undervoltage

lockout may be set with the UVLO pin at a voltage higher

than the internal undervoltage lockout. The duty cycle is

the fraction of time that the internal switch is on and is

determined by the input and output voltages:

DC

VV

VV

V

OUT

F

IN

SW

F

=

+

+

–

(~0.3V at maximum load). This leads to a minimum input

voltage of:

V

VV

DC

V

V

IN MIN

OUT

F

MAX

F

SW

()

–

=

+

+

The user defined undervoltage is set by a resistor divider

connected to the UVLO pin. The comparator pulls 3μA

from the pin when the UVLO pin is higher than 1.25V.

The hysteresis and minimum input voltage equations are

as follows:

VR

k

µA

VV

RR

R

HYS

IN MIN

=+

()

=

+

22

3 9

125

12

1

•.

.

()

R2

R1

UVLO

3513 A1

38

OPERATION