LT3840

13

3840fa

For more information www.linear.com/LT3840

operaTion

Soft-Start/Output Voltage Tracking

The soft-start function controls the slew rate of the power

supply output voltage during start-up. A controlled output

voltagerampminimizesoutputvoltageovershoot,reduces

sequencing.

TK/SS is an additional input to the error ampli-

fier (EA). The voltage control loop regulates the out-

TK/SS. An internal current source and a capacitor on the

pin program the output voltage ramp time. Drive the pin

with a voltage to use the output voltage tracking function

for supply sequencing.

The TK/SS voltage is clamped to a diode above the FB volt-

age, therefore, during a short-circuit the TK/SS voltage is

pulled low because the FB voltage is low. Once the short

has been removed the FB voltage starts to recover. The

soft-startcircuittakescontroloftheoutputvoltageslewrate

once the FB voltage has exceeded the slowly ramping TK/

SSvoltage,reducingtheoutputvoltageovershootthrough

a short-circuit recovery. During a fault condition such as

UVLO, OVLO or overtemperature, the soft-start capacitor

is discharged. If unused, the pin can be left open and the

internal current source will pull the pin voltage above the

soft-start operating range. Figure 5 is a functional block

diagram of the LT3840 soft-start/tracking function.

Power Good and Output Overvoltage Protection

When FB is within range of its regulated value an internal

N-channel MOSFET, on the PG pin, is turned off allowing

an external resistor to pull PG high. Power Good is valid

voltage is above 2.5V.

TheLT3840outputovervoltageprotectionfeaturedisables

the synchronous buck controller switching when the FB

pin exceeds its regulated value by a given amount (see

Electrical Specification table). When this event occurs the

PG pin voltage is pulled low.

Input Overvoltage Lockout

The LT3840 is capable of withstanding input voltage

transients up to 80V. When the voltage on the AUXVIN

pin exceeds 60V the auxiliary bias switching regulator is

disabled.

Figure 5. Soft-Start and Output Voltage Tracking Functional Block Diagram and Transfer Curve

FAULT

–

+

+

FB

TK/SS

EA

TRANSFER FUNCTION

FB vs TK/SS

0.1

0.2

0.4

0.3

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

TK/SS (V)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4

3840 F05