LTC1928-5
APPLICATIONS INFORMATION
U
W U U
A low ESR ceramic capacitor is recommended for the
flying capacitor CFLY with a value of 0.47µF. At low load or
high VIN a smaller capacitor could be used to reduce ripple
R
R
CPO
V
DROPOUT
CPO
V
OUT
+
–
V
DROPOUT
+
IOUT
2V
C
IN
CPO
on CPO which would reflect as lower ripple on VOUT
.
–
If a minimum enable time is required, the CPO output filter
capacitor should be at least 2× the VOUT filter capacitor.
When the LDO is first enabled, the CPO capacitor will
dump a large amount of charge into the VOUT capacitor. If
the drop in the CPO voltage falls below 1.45VIN the LDO
will be disabled and the CPO voltage will be required to
charge up to 1.75VIN to enable the LDO. The resulting
cycling extends the enable time.
19285 F02
Figure 2. Equivalent Circuit
External CPO Loading
TheCPOoutputcandriveanexternalload(forexample, an
LDO). The current required by this additional load will
reduce the available current from VOUT. If the external load
requires1mA, theavailablecurrentatVOUT willbereduced
by 1mA.
Output Ripple
The output ripple on CPO includes a spike component
from the charge pump switches and a droop component
whichisdependentontheloadcurrentandthevalueofC3.
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. CCPO should be increased for high load
currents to minimize the droop component. Ripple com-
ponents on CPO are greatly reduced at VOUT by the LDO,
however, COUT should also be a low ESR capacitor to
improve filtering of the CPO noise.
Short-Circuit and Thermal Protection
VOUT 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 conditions
remain in place, the part will cycle between the shutdown
and enabled states.
Shutdown
When CN/SHDN = 0V, the part will be in shutdown, the
supply current will be <8µA and VOUT will be shorted to
ground through a 160Ω switch. In addition, CPO will be
high impedance and disconnected from VIN and
CN/SHDN.
Capacitor Selection
For best performance it is recommended that low ESR
ceramic capacitors be used to reduce noise and ripple.
COUT must be ≥2µF and CCPO must be equal to or greater
than COUT. CIN 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. CIN should be ≥2µF and a spike
reducing resistor of 2.2Ω may be required between VIN
and the supply.
Shutdown is achieved by internally sampling the
CN/SHDN pin for a low voltage. Time between shutdown
samples is about 30µs. During the sample time the charge
pump switches are disabled and CN/SHDN must be pulled
to ground within 400ns. A resistor value between 100Ω
and1kisrecommended.Parasiticleadcapacitanceshould
be minimized on the CN/SHDN pin.
6
Linear [ Linear ]
