LT3513

11

3513fa

The LT3513 is a highly integrated power supply IC contain-

ing four separate switching regulators and a low dropout

linear regulator (LDO). Switching regulator 1 is a step-

down 2.5A regulator with inductor current sense and an

integrated boost Schottky diode. Switching regulator 2 can

be configured as a step-up or SEPIC converter and has a

1.2A switch. Switching regulator 3 consists of a step-up

regulator with a 0.25A switch as well as an integrated

Schottky diode. Switching regulator 4 is a negative regula-

tor with a switch current limit of 0.25A and an integrated

Schottky diode. Linear regulator 5 is capable of providing

8mA of current to the base of an external NPN transistor.

The regulators share common circuitry including input

source, voltage reference and master oscillator. Operation

can be best understood by referring to the Block Diagram

as shown in Figure 1.

If the RUN-SS1 pin is pulled to ground, the LT3513 is shut

An internal 2μA current source charges the external soft-

start capacitor, generating a voltage ramp at this pin. If the

RUN-SS1 pin exceeds 0.6V, the internal bias circuits turn

on, including the internal regulator, reference and 2MHz

master oscillator. The master oscillator generates four

clock signals, one for each of the switching regulators.

Switching regulator 1 will only begin to operate when the

which must be tied to the BIAS pin. When BIAS reaches 2.8V,

the NPNs pulling down on the RUN-SS2 and RUN-SS3/4

pins turns off, allowing an internal 2μA current source

to charge the external capacitors tied to RUN-SS2 and

RUN-SS3/4 pins. When the voltage on RUN-SS2 reaches

0.8V, switcher 2 is enabled. Correspondingly, when the

voltage on RUN-SS3/4 reaches 0.8V, switchers 3 and 4

rate determined by the capacitors tied to the RUN-SS2 and

RUN-SS3/4 pins. When all four switching outputs reach

90% of their programmed voltages, the NPN pulling down

four outputs dropping below 10% of their programmed

when FB2 is at or above 90% of its regulated value. The

output is an open-collector transistor that is off when the

output is out of regulation, allowing an external resistor

to pull the pin high. This pin can be used with a P-chan-

nel MOSFET that functions as an output disconnect for

The four switchers are current mode regulators. Instead

of directly modulating the duty cycle of the power switch,

the feedback loop controls the peak current in the switch

during each cycle. Compared to voltage mode control, cur-

rent mode control improves loop dynamics and provides

cycle-by-cycle current limit.

(2a)

(2b)

Figure 2. LT3513 Power-Up Sequence. (Traces from Both Photos

are Synchnonized to the Same Trigger)

RUN-SS 2V/DIV

SS-234 2V/DIV

PGOOD 20V/DIV

5ms/DIV

3513 F02a

5ms/DIV

3513 F02b

OPERATION