9
LTC1682/LTC1682-3.3/LTC1682-5
The following formula can be used to find the maximum
output voltage that may be programmed for a given
minimum input voltage and output current load:
V
OUT(MAX)
= (2)(V
IN(MIN)
) – (I
OUT
)(R
CPO
) – V
DROPOUT
with the condition that (I
OUT
)(R
CPO
) < 0.55V
IN
.
Example:
V
IN(MIN)
= 3V
I
OUT
= 10mA
R
CPO(MAX)
= 20Ω
Max unloaded CPO voltage = 6V
Loaded CPO voltage = 6V – (10mA)(20Ω) = 5.8V
V
DROPOUT(MAX)
= 0.08V
V
OUT(MAX)
= (6V) – (0.2V) – (0.08V) = 5.72V
V
OUT
< 5.5V and (I
OUT
)(R
CPO
) < 0.55V
IN
, 0.2V < 1.65V.
For minimum noise applications, the LDO must be kept out
of dropout to prevent CPO noise from coupling into V
OUT
.
External CPO Loading
The CPO output can drive an external load (an LDO, for
example). The current required by this additional load will
reduce the available current from V
OUT
. If the external load
requires 5mA, then the maximum available current at V
OUT
will be reduced by 5mA.
Short-Circuit and Thermal Protection
V
OUT
can be shorted to ground indefinitely. Internal cir-
cuitry will limit the output current. If the junction tempera-
ture exceeds 150°C, the part will shut down. Excessive
power dissipation due to heavy loads will also cause the
part to shut down when the junction temperature exceeds
150°C. The part will become enabled when the junction
temperature drops below 140°C. If the fault condition
remains in place, the part will cycle between the shutdown
and enabled states.
Capacitor Selection
For best performance it is recommended that low ESR
capacitors be used for C2, C3 and C4 in Figure 1 to
reduce noise and ripple. C2 must be ≥2µF and C3 must
be equal to or greater than C2. C4 is dependent on the
source impedance. The charge pump demands large
APPLICATIONS INFORMATION
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instantaneous currents which may induce ripple onto
a common voltage rail. C4 should be ≥2µF and a spike
reducing resistor of 2.2Ω may be required between
V
IN
and the supply.
A low ESR ceramic capacitor is recommended for the
flying capacitor C1 with a value of 0.22µF. At low load or
high V
IN
a smaller capacitor could be used to reduce ripple
on CPO which would reflect as lower ripple on V
OUT
.
If a minimum enable time is required, the CPO output filter
capacitor should be at least 2× the V
OUT
filter capacitor.
When the LDO is first enabled, the CPO capacitor will
dump a large amount of charge into the V
OUT
capacitor. If
the drop in the CPO voltage falls below 1.45(V
IN
), the LDO
will be disabled and the CPO voltage will have to charge up
to 1.75(V
IN
) to enable the LDO. The resulting cycling
extends the enable time.
A 1nF filter capacitor for the LTC1682-3.3/LTC1682-5
should be connected between the FILT pin and ground for
optimum noise performance.
Output Ripple
The output noise and ripple on CPO includes a spike
component from the charge pump switches and a droop
component which is dependent on the load current and the
value of C3. The charge pump has been carefully designed
to minimize the spike component; however, low ESR
capacitors are essential to reduce the remaining spike
energy effect on the CPO voltage. C3 should be increased
for high load currents to minimize the droop component.
Ripple components on CPO are greatly reduced at V
OUT
by
the LDO; however, C2 should also be a low ESR capacitor
to improve filtering of the CPO noise.
Shutdown
When SHDN pin is pulled low (<0.4V), the part will be in
shutdown, the supply current will be < 5µA and V
OUT
will
be connected to ground through a 100Ω switch. In addi-
tion, CPO will be high impedance and disconnected from
V
IN
.
If shutdown is not required, connect SHDN to V
IN
which
will continuously enable the part.
