LT1129 Linear Technology, LT1129 Datasheet - Page 7

• Manufacturer Part Number: LT1129
• Description: Micropower Low Dropout Regulators with Shutdown
• Manufacturer: Linear Technology

PI FU CTIO S
Input Pin: Power is supplied to the device through the
input pin. The input pin should be bypassed to ground if
the device is more than 6 inches away from the main input
filter capacitor. In general, the output impedance of a
battery rises with frequency so it is advisable to include a
bypass capacitor in battery-powered circuits. A bypass
capacitor in the range of 1 F to 10 F is sufficient. The
LT1129 is designed to withstand reverse voltages on the
input pin with respect to both ground and the output pin.
In the case of a reversed input, which can happen if a
battery is plugged in backwards, the LT1129 will act as if
there is a diode in series with its input. There will be no
reverse current flow into the LT1129 and no reverse
voltage will appear at the load. The device will protect both
itself and the load.
Output Pin: The output pin supplies power to the load. An
output capacitor is required to prevent oscillations. See
the Applications Information section for recommended
value of output capacitance and information on reverse
output characteristics.
Shutdown Pin (SHDN): 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
TYPICAL PERFOR
–10
–15
–20
–25
–30
–5
U
0
–50
Load Regulation
V
*V
IN
I
–25
LOAD
ADJ
= V
LT1129-3.3
U
= V
OUT
= 1mA to 700mA
LT1129*
OUT
0
(NOMINAL) + 1V
TEMPERATURE (°C)
25
U
LT1129-5
50
75
W
100
A
1129 G09
125
U
CE
C
–0.05
–0.10
0.10
0.05
HARA TERISTICS
0.6
0.5
0
LT1129-5
Transient Response
0
V
C
C
50
IN
IN
OUT
= 6V
= 3.3 F
100 150
= 3.3 F
C
200
TIME ( s)
250
shutdown pin current with the pin pulled to ground will be
6 A. The shutdown pin is internally clamped to 7V and –
0.6V (one V
directly by 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 microamperes. Pull-up current must be limited to
a maximum of 20mA. A curve of shutdown pin input
current as a function of voltage appears in the Typical
Performance Characteristics. 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.
Sense Pin: For fixed voltage versions of the LT1129
(LT1129-3.3, LT1129-5) the sense pin is the input to the
error amplifier. Optimum regulation will be obtained at the
point where the sense pin is connected to the output pin.
For most applications the sense pin is connected directly
to the output pin at the regulator. In critical applications
small voltage drops caused by the resistance (R
traces between the regulator and the load, which would
normally degrade regulation, may be eliminated by con-
necting the sense pin to the output pin at the load as shown
in Figure 1 (Kelvin Sense Connection). Note that the
voltage drop across the external PC traces will add to the
dropout voltage of the regulator. The sense pin bias
300
350
LT1129/LT1129-3.3/LT1129-5
400
450
BE
1129 G31
). This allows the shutdown pin to be driven
500
–0.1
–0.2
0.2
0.1
0.7
0.5
0.3
0.1
0
0
LT1129-5
Transient Response
V
C
C
0.2
IN
IN
OUT
= 6V
= 3.3 F
0.4 0.6
= 47 F
0.8
TIME (ms)
1.0
1.2
1.4
P
1.6
) of PC
1.8
1129 G32
7
2.0