5

LTC1477/LTC1478

Input TTL-CMOS Converter

The LTC1477 enable input is designed to accommodate a

wide range of 3V and 5V logic families. The input threshold

voltage is approximately 1.4V with 100mV of hysteresis.

The input enables the bias generator, the gate charge

pump and the protection circuitry. Therefore, when the

enable input is turned off, the entire circuit is powered

down and the supply current drops below 1

µA.

Ramped Switch Control

The LTC1477 gate charge pump includes circuitry which

ramps the NMOS switch on slowly (1ms typical rise time)

but turns it off much more quickly (typically 20

µs).

Bias, Oscillator and Gate Charge Pump

When the switch is enabled, a bias current generator and

high frequency oscillator are turned on. The on-chip

capacitive charge pump generates approximately 12V of

the power supply. No external 12V supply is required to

switch the output.

Switch Protection

Two levels of protection are designed into the power

switch in the LTC1477. The switch is protected against

accidental short circuits with a current limit circuit which

limits the output current to typically 2A when the output is

shorted to ground. The LTC1477 also has thermal shut-

down set at approximately 130

°C which limits the power

dissipation to safe levels.

LTC1478 Operation

The LTC1478 dual protected switch can be thought of as

two independent LTC1477 single protected switches. The

input supply voltages may be from separate power sources.

The ground connection, however, is common to both

channels and must be connected to the same potential.

(LTC1477 or single channel of LTC1478)

CONNECTED

DISCONNECTED

DISCONNECTED

0.07

Ω

0.09

Ω

0.12

Ω

2A

1.5A

0.85A

Note: 5V Operation

Note that there is an inverse relationship between output

current limit and switch resistance. This allows the tailor-

TTL-TO-CMOS

CONVERTER

OSCILLATOR

AND BIAS

CHARGE

PUMP

GATE CHARGE

AND

DISCHARGE

CONTROL LOGIC

CURRENT LIMIT

AND THERMAL

SHUTDOWN

EN

LTC1477/1478 • BD01

The LTC1477 is designed to current limit at approximately

the input power supply. It is possible however, to reduce

this current by selectively disconnecting two of the four

current limit