6

LTC3408

3408f

FU CTIO AL DIAGRA

+

–

+

–

+

–

BCMP

EA

P-CHANNEL

FB

+

–

6

5

RUN

OSC

SLOPE

COMP

OSC

FREQ

1.2V

0.85V

SLEEP

REF

8

1

360k

180k

BURST

S

R

RS LATCH

SWITCHING

LOGIC

AND

BLANKING

CIRCUIT

ANTI-

SHOOT-

THRU

Q

Q

5

Ω

2

7

SW

4

GND

3408 BD

3

9

Main Control Loop

The LTC3408 uses a constant frequency, current mode step-

down architecture. The main (P-channel MOSFET), syn-

chronous (N-channel MOSFET) and bypass (P-channel

MOSFET) switches are internal. During normal operation,

the internal main switch is turned on each cycle when the

oscillator sets the RS latch, and turned off when the cur-

trolled by the output of error amplifier EA. When the load

current increases, it causes a slight decrease in the feed-

back voltage, FB, relative to the external reference, which

in turn, causes the EA amplifier’s output voltage to increase

until the average inductor current matches the new load cur-

rent. While the main switch is off, the synchronous switch

is turned on until the beginning of the next clock cycle.

The LTC3408 operates in forced continuous mode where

the inductor current is constantly cycled. In this mode, the

output voltage can respond quickly to the external refer-

ence voltage by sourcing or sinking current as needed.

Controlling the Output Voltage

The output voltage can be dynamically programmed from

from REF is internally set to 3, the corresponding input

during operation by driving REF with an external DAC.

When REF exceeds 1.2V, a 0.08

ΩinternalbypassP-channel

drop across the inductor and the main switch.

Short-Circuit Protection

A current sense comparator monitors the current across

the bypass P-channel MOSFET with a trip current of about

to ground, the bypass P-channel MOSFET is immediately

turned off. The propagation delay of the current sensing

turning off the bypass P-channel MOSFET is approxmately

100ns. Once the bypass P-channel MOSFET is off for about

10

µs to 20µs, it is allowed to turn back on. The initial

current limit is then lowered to about 1.6A after the first

current limit trip. If the short to ground persists, the cur-

rent comparator will trip at the lower current limit, turning

†

10

µF

CER

2.7V

TO 5V

LTC3408

RUN

REF

REF

4.7

µH*

3403 F01

*

**

†

MURATA LQH32CN4R7M11

TAIYO YUDEN JMK212BJ475MG

TAIYO YUDEN JMK212BJ106MN

SW

GND

4.7

µF

CER

3

600mA

Figure 1. Typical Application