Datasheet

4
LTC1911-1.5/LTC1911-1.8
1911f
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Start-Up Time
vs Soft-Start Capacitor
Output Ripple
vs Output Load Current
Output Current Transient Response
Line Transient Response
SOFT-START CAPACITOR (nF)
0.1
0.1
START-UP TIME (ms)
1
10
100
110
1911 G10
100
T
A
= –40°C
T
A
= 25°C
T
A
= 85°C
V
IN
= 3.6V
4V
3V
V
IN
500mV/DIV
V
OUT
20mV/DIV
I
OUT
= 225mA 20µs/DIV 1911 G14
250mA
25mA
I
OUT
V
OUT
20mV/DIV
V
IN
= 3.6V 10µs/DIV 1911 G13
V
IN
(Pin 1): Input Supply Voltage. V
IN
may be between
2.7V and 5.5V. Suggested bypass for V
IN
is a 10µF (1µF
min) ceramic low ESR capacitor.
C2
+
(Pin 2): Flying Capacitor Two Positive Terminal.
C2
–
(Pin 3): Flying Capacitor Two Negative Terminal.
GND (Pin 4): Ground. Connect to a ground plane for best
performance.
C1
–
(Pin 5): Flying Capacitor One Negative Terminal.
V
OUT
(Pin 6): Regulated Output Voltage. V
OUT
is discon-
nected from V
IN
during shutdown. Bypass V
OUT
with a
≥10µF ceramic low ESR capacitor (4µF min, ESR < 0.1Ω
max).
C1
+
(Pin 7): Flying Capacitor One Positive Terminal.
UU
U
PI FU CTIO S
SS/SHDN (Pin 8): Soft-Start/Shutdown Control Pin. This
pin is designed to be driven with an external open-drain
output. Holding the SS/SHDN pin below 0.3V will force
the LTC1911-X into shutdown mode. An internal pull-up
current of 2µA will force the SS/SHDN voltage to climb to
V
IN
once the device driving the pin is forced into a Hi-Z
state. To limit inrush current on start-up, connect a
capacitor between the SS/SHDN pin and GND. Capaci-
tance on the SS/SHDN pin will limit the dV/dt of the pin
during turn on which, in turn, will limit the dV/dt of V
OUT
.
By selecting an appropriate soft-start capacitor, the user
can control the inrush current for a known output capaci-
tor during turn-on (see Application Information). If nei-
ther of the two functions are desired, the pin may be left
floating or tied to V
IN
.
LTC1911-1.8 Output Voltage Ripple
V
OUT
50mV/DIV
2-TO-1 MODE
V
IN
= 5V
I
OUT
= 250mA 100ns/DIV 1911 G12
ALL WAVEFORMS AC COUPLED
OUTPUT LOAD CURRENT (mA)
0
0
OUTPUT RIPPLE (mV
P-P
)
5
10
15
20
30
50
100 150 200
1911 G11
250 300
25
C
OUT
= 4.7µF
C
OUT
= 10µF
C
OUT
= 22µF
V
OUT
50mV/DIV
3-TO-2 MODE
V
IN
= 3.6V
V
OUT
50mV/DIV
1-TO-1 MODE
V
IN
= 2.7V
V
IN
(V)
2.5
1.40
FREQUENCY (MHz)
1.45
1.50
1.55
1.60
3.0 3.5 4.0 4.5
1911 G15
5.0 5.5
T
A
= –40°C
T
A
= 25°C
T
A
= 85°C
Oscillator Frequency
vs Input Supply Voltage