LTC1647-2CS8 datasheet(9/20 Pages) LINER

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LTC1647-1/LTC1647-2/LTC1647-3

VCC Selection Circuit

The LTC1647-3 features separate supply inputs (VCC1 and

VCC2) for each channel. The reference and charge pump

circuit draw supply current from the higher of the two

supplies. An internal VCC selection circuit detects and

makes the power connection automatically. This allows a

3V channel to have standard MOSFET gate overdrive when

the other channel is 5V. An internal Zener clamps GATE

about 15V above VCC.

If both supplies are connected together (internally for

LTC1647-1 and LTC1647-2 or externally for LTC1647-3),

the reference and charge pump circuit draw equal current

from both pins.

Electronic Circuit Breaker

Each channel of the LTC1647 features an electronic circuit

breaker to protect against excessive load current and

short-circuits. Load current is monitored by sense resis-

tor R1 as shown in Figure 1. The circuit breaker threshold,

VCB, is 50mV and it exhibits a response time, tFAULT, of

approximately 300ns. If the voltage between VCC and

SENSE exceeds VCB for more than tFAULT, the circuit

breaker trips and immediately pulls GATE low with a 50mA

current sink. The MOSFET turns off and FAULT pulls low.

The circuit breaker is cleared by pulling the ON pin low for

a period of at least tRESET (50µs). A timing diagram of these

events is shown in Figure 2.

The value of the sense resistor R1 is given by

R1 = VCB/ITRIP(Ω)

where VCB is the circuit breaker trip voltage (50mV) and

ITRIP is the value of the load current at which the circuit

breaker trips. Kelvin-sense layout techniques between the

sense resistor and the VCC and SENSE pins are highly

recommended for proper operation.

LTC1647-3

APPLICATIO S I FOR ATIO

BLOCK DIAGRA S

–

µs

FILTER

–

1.21V

50mV

CHANNEL ONE

CHANNEL TWO

(DUPLICATE OF CHANNEL ONE)

µA

–

µA

FAULT

2.45V

UVL

CHARGE

PUMP

REFERENCE

1.21V

SENSE 2

ON2

FAULT 2

1647-1/2/3 BD3

VCC2

12 GATE 2

GND

VCC

SELECTION

SENSE 1

13 GATE 1

FAULT 1

ON1

VCC1

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