LTC1928-5
6
19285fa
For more information www.linear.com/LTC 1928-5
APPLICATIONS INFORMATION
ternal reference and resistor divider. The LDO requires a
capacitor on V
OUT
for stability and improved load transient
response. A low ESR capacitor of ≥2µF should be used.
Maximum I
OUT
Calculations
The maximum available current can be calculated based
on the open circuit CPO voltage, the dropout voltage of
the LDO and the effective output resistance of the charge
pump. The open circuit CPO voltage is approximately 2V
IN
(see Figure 2).
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 conditions
remain in place, the part will cycle between the shutdown
and enabled states.
Capacitor Selection
For best performance it is recommended that low ESR
ceramic capacitors be used to reduce noise and ripple.
C
OUT
must be ≥2µF and C
CPO
must be equal to or greater
than C
OUT
. C
IN
is dependent on the input power supply
source impedance. The charge pump demands large
instantaneous currents which may induce ripple onto
a common voltage rail. C
IN
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 C
F LY
with a value of 0.47µF. 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 1.45V
IN
the LDO
will be disabled and the CPO voltage will be required to
charge up to 1.75V
IN
to enable the LDO. The resulting
cycling extends the enable time.
Output Ripple
The output 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 mini
-
mize the spike component, however, low ESR capacitors
are essential to reduce the remaining spike energy effect
on the CPO voltage. C
CPO
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, C
OUT
should also be a low ESR capacitor
to improve filtering of the CPO noise.
Example:
V
IN
= 3V
V
OUT
= 5V
R
CPO
= 30Ω
Maximum unloaded CPO voltage = 2V
IN
= 6V
V
DROPOUT(MAX)
= 100mV
I
OUT(MAX)
= (2V
IN
– V
DROPOUT(MAX)
– V
OUT
)/R
CPO
= (6V – 0.1V – 5V)/30Ω = 30mA
V
CPO
must be greater than 1.45V
IN
= 4.35V. To confirm
this, calculate V
CPO
:
V
CPO
= 6V – (30mA • 30Ω) = 5.1V
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 (for example,
an LDO). The current required by this additional load will
reduce the available current from V
OUT
. If the external
load requires 1mA, the available current at V
OUT
will be
reduced by 1mA.
Short-Circuit and Thermal Protection
V
OUT
can be shorted to ground indefinitely. Internal circuitry
will limit the output current. If the junction temperature
Figure 2. Equivalent Circuit
+
–
+
–
R
CPO
R
DROPOUT
V
DROPOUT
C
CPO
V
CPO
V
OUT
I
19285 F02
2V
IN
