LTC1645CS View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1645CS

Dual-Channel Hot Swap Controller/Power Sequencer

Linear Technology 

APPLICATIO S I FOR ATIO

LTC1645

VIN2

5V/DIV

VIN1

5V/DIV

VOUT2

5V/DIV

VOUT1

5V/DIV

TIMER

2V/DIV

RESET

5V/DIV

Figure 16. Ramping 3.3V and 2.5V Up and Down Together

This circuit guarantees that: (1) VOUT1 never exceeds

VOUT2 by more than 1.2V, and (2) VOUT2 is never greater

than VOUT1 by more than 0.4V. On power-up, VOUT1 and

VOUT2 ramp up together. On power-down, the LTC1645

turns off Q1 and Q2 simultaneously. Charge remains

stored on CLOAD1 and CLOAD2 and the output voltages will

vary depending on the loads. D1 and D2 turn on at ≈ 1V

(≈ 0.5V each), ensuring condition 1 is satisfied, while D3

prevents violations of condition 2. Different diodes may be

necessary for different output voltage configurations.

Barring an overvoltage condition at the input(s), the only

time these diodes might conduct current is during a

power-down event, and then only to discharge CLOAD1 or

CLOAD2. In the case of an input overvoltage condition that

causes excess current to flow, the circuit breaker will trip

if the current limit level is set appropriately.

Figure 17 shows an application circuit where VOUT1

ramps up before VOUT2. VOUT1 is initially discharged and

D1 is reverse-biased, thus the voltage at the ON pin is

determined only by VCC1 through the resistor divider R1

and R2. The voltage at the ON pin exceeds 0.8V if VCC1 is

above 4.6V and VOUT1 begins to ramp up after a timing

cycle. As VOUT1 ramps up, D1 becomes forward-biased

and pulls the ON pin above 2V when VOUT1 ≈ 4.5V. This

turns on GATE2 and VOUT2 ramps up. The FB comparator

monitors VOUT2, and the spare comparator monitors

VOUT1 with RHYST creating ≈50mV of hysteresis.

Power Supply Multiplexer

Using back-to-back FETs, the LTC1645 can Hot Swap two

supplies to the same output, automatically selecting the

primary supply if present or the secondary supply if the

primary supply is not available. Referring to Figure 18, a

diode-or circuit provides power to the LTC1645 if either

supply is up. Schottky diodes are used to prevent the

voltage at VCC1 from approaching the undervoltage lock-

out threshold. This application assumes that if a supply is

not present, the supply input is floating.

If only the 3.3V supply is present, the voltage at the COMP+

pin is below the trip point and COMPOUT pulls the base of

Q3 low, allowing the GATE1 pin to ramp up normally. The

voltage at the ON pin exceeds 0.8V if the 3.3V supply is

greater than 3V, ramping up GATE1 and turning on Q1A and

Q1B. The ON pin does not exceed 2V (unless the 3.3V supply

exceeds 7.5V!), keeping GATE2 low and Q2A and Q2B off.

If only the 5V supply is present or if both supplies are

present, the COMP+ pin is above 1.238V and COMPOUT

allows the base of Q3 to be pulled high by R2. This turns

Q3 on, keeping GATE1 low and Q1A and Q1B off. The

voltage at the ON pin is pulled above 2V by R1 and GATE2

turns Q2A and Q2B on.

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