Datasheet

5
LTC1477/LTC1478
OPERATION
U
(LTC1477 or single channel of 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 1µA.
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 20µs).
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
gate drive for the internal low R
DS(ON)
NMOS switch from
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)
BLOCK DIAGRAM
W
APPLICATIONS INFORMATION
WUU
U
Table 1. Effects of Disconnecting V
IN2
and V
IN3
ALL V
IN
PINS V
IN3
V
IN2
AND V
IN3
CONNECTED DISCONNECTED DISCONNECTED
R
DS(ON)
0.07Ω 0.09Ω 0.12Ω
I
LIMIT
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
V
INS
V
IN1
V
IN2
V
IN3
V
OUT
LTC1477/1478 • BD01
Tailoring I
LIMIT
and R
DS(ON)
for Load Requirements
The LTC1477 is designed to current limit at approximately
2A during a short circuit with all the V
IN
pins connected to
the input power supply. It is possible however, to reduce
this current by selectively disconnecting two of the four
power supply pins (V
IN2
and V
IN3
). Table 1 lists the effects
of disconnecting these pins on R
DS(ON)
and short-circuit
current limit