Datasheet

8
LTC1911-1.5/LTC1911-1.8
1911f
4.7nF capacitor on SS/SHDN results in a 3ms ramp time
from 0.6V to 1.9V on the pin. If C
OUT
is 10µF, the 3ms V
REF
ramp time results in an average C
OUT
charge current of
only 6mA (see Figure 2).
Low Current Burst Mode Operation
To improve efficiency at low output currents, a Burst Mode
function was included in the design of the LTC1911. An
output current sense circuit is used to detect when the
required output current drops below 30mA typ. When this
occurs, the oscillator shuts down and the part goes into a
low current operating state. The LTC1911 will remain in
the low current operating state until V
OUT
has dropped
enough to require another burst of current. Unlike tradi-
tional charge pumps who’s burst current is dependant on
many factors (i.e., supply, switch strength, capacitor
selection, etc.), the LTC1911 burst current is set by the
burst threshold. This means that the output ripple voltage
during Burst Mode operaton will be fixed and is typically
5mV for C
OUT
= 10 µ F.
Short-Circuit/Thermal Protection
The LTC1911 has built-in short-circuit current limiting as
well as overtemperature protection. During short-circuit
conditions it will automatically limit its output current to
approximately 600mA. The LTC1911 will shut down if the
junction temperature exceeds approximately 160°C. Un-
der normal operating conditions, the LTC1911 should not
go into thermal shutdown but it is included to protect the
IC in cases of excessively high ambient temperatures, or
in cases of excessive power dissipation inside the IC (i.e.,
overcurrent or short circuit). The charge transfer will
reactivate once the junction temperature drops back to
approximately 150°C. The LTC1911 can cycle in and out
of thermal shutdown indefinitely without latch-up or
damage until the fault condition is removed.
V
OUT
Ripple and Capacitor Selection
The type and value of capacitors used with the
LTC1911 determine several important parameters such
as regulator control loop stability, output ripple and
charge pump strength.
The value of C
OUT
directly controls the amount of output
ripple for a given load current. Increasing the size of C
OUT
will reduce the output ripple.
APPLICATIO S I FOR ATIO
WUUU
Figure 2. Shutdown/Soft-Start Operation
SS/SHDN
C
SS
ON OFF V
CTRL
6
8
V
OUT
R
LOAD
LTC1911
C
OUT
(2a)
V
CTRL
2V/DIV
V
OUT
1V/DIV
C
SS
= 0nF 2ms/DIV 1911 F02b
C
OUT
= 10µF
R
LOAD
= 10Ω
(2b)
V
CTRL
2V/DIV
V
OUT
1V/DIV
C
SS
= 4.7nF 2ms/DIV 1911 F02c
C
OUT
= 10µF
R
OUT
= 10Ω
(2c)