LT1528 Linear Technology, LT1528 Datasheet - Page 7

• Manufacturer Part Number: LT1528
• Description: 3A Low Dropout Regulator for Microprocessor Applications
• Manufacturer: Linear Technology

APPLICATIONS
SHDN (Pin 4): This pin is used to put the device into
shutdown. In shutdown the output of the device is turned
off. This pin is active low. The device will be shut down if
the Shutdown pin is actively pulled low. The Shutdown pin
current with the pin pulled to ground will be 60 A. The
Shutdown pin is internally clamped to 7V and – 0.6V (one
V
5V logic or by open collector logic with a pull-up resistor.
The pull-up resistor is only required to supply the leakage
current of the open collector gate, normally several micro-
amperes. Pull-up current must be limited to a maximum
of 5mA. A curve of Shutdown pin input current as a
function of voltage appears in the Typical Performance
Characteristics section. If the Shutdown pin is not used it
can be left open circuit. The device will be active output on
if the Shutdown pin is not connected.
V
input pin. The input pin should be bypassed to ground if
The LT1528 is a 3A low dropout regulator optimized for
microprocessor applications. Dropout voltage is only 0.6V
at 3A output current. With the Sense pin shorted to the
Output pin, the output voltage is set to 3.3V. The device
operates with a quiescent current of 400 A. In shutdown,
the quiescent current drops to only 125 A. The LT1528
incorporates several protection features, including protec-
tion against reverse input voltages. If the output is held at
the rated output voltage when the input is pulled to ground,
the LT1528 acts like it has a diode in series with its output
and prevents reverse current flow.
Adjustable Operation
The LT1528 can be used as an adjustable regulator with an
output voltage range of 3.3V to 14V. The output voltage is
set by the ratio of two external resistors as shown in
Figure 2. The device servos the output voltage to maintain
the voltage at the Sense pin at 3.3V. The current in R1 is
then equal to 3.3V/R1. The current in R2 is equal to the sum
of the current in R1 and the Sense pin current. The Sense
pin current, 130 A at 25 C, flows through R2 into the
Sense pin. The output voltage can be calculated using the
PIN
BE
IN
U
). This allows the Shutdown pin to be driven directly by
(Pin 5): Power is supplied to the device through the
FUNCTIONS
U
U
U
INFORMATION
U
W
U
the device is more than six inches away from the main
input filter capacitor. The LT1528 is designed to withstand
reverse voltages on the input pin with respect to ground
and the Output pin. In the case of reversed input, the
LT1528 will act as if there is a diode in series with its input.
There will be no reverse current flow into the LT1528 and
no reverse voltage will appear at the load. The device will
protect both itself and the load.
formula in Figure 2. The value of R1 should be less than
330 to minimize errors in the output voltage caused by
the Sense pin current. Note that in shutdown the output is
turned off and the divider current will be zero. Curves of
Sense Pin Voltage vs Temperature and Sense Pin Current
vs Temperature appear in the Typical Performance Char-
acteristics section.
V
IN
+
V
Figure 1. Kelvin Sense Connection
IN
Figure 2. Adjustable Operation
5
4
V
V
I
OUTPUT RANGE = 3.3V TO 14V
SENSE
5
4
OUT
SENSE
IN
SHDN
IN
SHDN
= 3.3V 1 +
= 130 A AT 25 C
= 3.3V
LT1528
LT1528
GND
GND
3
R2
R1
3
SENSE
SENSE
+ (I
OUT
OUT
SENSE
1
2
1
2
+ R2)
R
R
P
P
R2
R1
+
+
LT1528
LT1528 • F01
LT1528 • F02
LOAD
V
OUT
7