LTC1645CS datasheet(17/24 Pages) LINER

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LTC1645

APPLICATIO S I FOR ATIO

Using the LTC1645 as a Linear Regulator

This application uses the LTC1645 to Hot Swap one

primary supply and generate a secondary low dropout

regulated supply. Figure 19 shows how to switch a 5V

supply and create a 3.3V supply using the spare compara-

tor and one additional transistor. The COMP+ pin is used

to monitor the 3.3V output. As the voltage on the gate of

Q2 increases, the 3.3V output increases. At the 3.3V

threshold the spare comparator trips. The COMPOUT pin

goes high which turns on Q3. This lowers the voltage on

the gate of Q2. This feedback loop is compensated by

capacitors C1 and C2 and resistor R1. When power is first

applied, the FB pin is low and RESET holds one side of C2

low, slowing the ramp-up of VOUT2. As VOUT2 exceeds

2.75V, RESET releases to allow improved loop transient

response. Figure 20 shows the load transient response

and voltage ripple of the generated supply.

IRF7413

0.01

Ω*

0.01

Ω*

IRFZ24

Ω

10k

Ω

200k

PN2222

1.5k

12.1k

10k

2.49k

0.1

µF

25V

0.033

µF

0.1

µF

25V

0.33

µF

470

µF**

×2

CLOAD1

BOTH CURRENT LIMITS: 5A

VCC1

FAULT

SENSE1

TIMER

GND

GATE2

VCC2

VIN

VOUT2

3.3V

2.5A

VOUT1

2.5A

LTC1645

(14-LEAD)

COMP+

COMPOUT

SENSE2

GATE1

1645 F19

RESET

LRF1206-01-R010-J (IRC)

T510X477K006AS (KEMET)

Figure 19. Switching 5V and Generating 3.3V

VOUT2

0.1V/DIV

IOUT2

1A/DIV

2.5A

0.5A

Figure 20. Load Transient Response and Voltage Ripple

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