LTC1503-1.8/LTC1503-2
APPLICATIO S I FOR ATIO
S5
φ2
V
IN
C1
(EXTERNAL)
S4
C1
–
φ2
C1
+
S1
φ1
V
OUT
S7
φ2
S2
φ1
C2
+
C2
(EXTERNAL)
S6
C2
–
φ2
S3
φ1
GND
1503-1.8/2 F01b
Figure 1b. Step-Down Charge Pump in 3-to-2 Mode
In 1-to-1 mode, switch S1 and S2 are connected in series
between V
IN
and V
OUT
as needed to boost V
OUT
back into
regulation (see Figure 1c). The REG ENABLE signal from
the main comparator (COMP1) controls switches S1 and
S2 directly. Since all of the V
OUT
current is sourced from
V
IN
, the efficiency in 1-to-1 mode is approximately equal
to that of a linear regulator.
S2
V
IN
C1
(EXTERNAL)
C1
–
1503-1.8/2 F01c
C1
+
S1
V
OUT
Figure 1c. Step-Down Charge Pump in 1-to-1 Mode
Mode Selection and Mode Skipping
The optimal step-down conversion mode is chosen based
on V
IN
to V
OUT
differential voltage and output load condi-
tions. Two internal comparators are used to select the
default step-down mode based on the V
IN
and V
OUT
voltage. A separate comparator (COMP2) is used to sense
a droop on V
OUT
due to a heavy output load and force the
charge pump to skip to a higher output current mode to
U
maintain regulation. This will only occur as V
IN
/V
OUT
nears
a 3-to-2 or 1-to-1 transition point. For example, under light
load conditions, the LTC1503-X can operate in 2-to-1
mode when V
IN
equals 4.1V with greater than 90% effi-
ciency. However, when the load is increased, the part can
no longer supply enough output current in 2-to-1 mode to
maintain regulation. This causes V
OUT
to droop below the
regulation point until COMP2 trips and forces the part to
skip from 2-to-1 mode to 3-to-2 mode. The COMP2
threshold is about 17mV (V
OUT
referred) below the main
comparator regulation point. Hysteresis in COMP2 will
force the part to transition in and out of mode skipping.
This will result in a slight V
OUT
decrease of approximately
20mV under mode skipping conditions.
Shutdown/Soft-Start Operation
The SHDN/SS pin is used to implement both low current
shutdown and soft-start. The soft-start feature limits
inrush currents when the regulator is initially powered up
or taken out of shutdown. Forcing a voltage lower than
0.35V (typ) will put the part into shutdown mode. Shut-
down mode disables all control circuitry and forces the
charge pump V
OUT
into a high impedance state. A 2µA pull-
up current on the SHDN/SS pin will force the part into
active mode if the pin is left floating or is driven with an
open-drain output that is in a high impedance state. If the
pin is not driven with an open-drain device, it must be
forced to a logic high voltage of 2.2V (min) to ensure
proper V
OUT
regulation. The SHDN/SS pin should not be
driven to a voltage higher than V
IN
.
To implement soft-start, the SHDN/SS pin must be driven
with an open-drain device and a capacitor must be
connected from the SHDN/SS pin to GND. Once the open-
drain device is turned off, a 2µA pull-up current will begin
charging the external SS capacitor and force the voltage
on the pin to ramp towards V
IN
. As soon as the SHDN
threshold is reached (0.35V typ), the internal reference
voltage which controls the V
OUT
regulation point will
follow the ramp voltage on the SHDN/SS pin (minus a
0.35V offset to account for the SHDN threshold) until the
reference reaches its final band gap voltage. This occurs
when the voltage on the SHDN/SS pin reaches
W
U U
7
LINER [ LINEAR TECHNOLOGY ]
