LTC1928-5 LINER [Linear Technology], LTC1928-5 Datasheet - Page 6

• Manufacturer Part Number: LTC1928-5
• Description: Doubler Charge Pump with Low Noise Linear Regulator in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1928-5
6
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
requires 1mA, the available current at V
by 1mA.
Short-Circuit and Thermal Protection
V
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.
Capacitor Selection
For best performance it is recommended that low ESR
ceramic capacitors be used to reduce noise and ripple.
C
than C
source impedance. The charge pump demands large
instantaneous currents which may induce ripple onto a
common voltage rail. C
reducing resistor of 2.2 may be required between V
and the supply.
OUT
OUT
must be 2 F and C
can be shorted to ground indefinitely. Internal cir-
OUT
. C
IN
is dependent on the input power supply
+
–
Figure 2. Equivalent Circuit
R
CPO
2V
U
IN
IN
INFORMATION
V
U
CPO
CPO
should be 2 F and a spike
C
must be equal to or greater
CPO
R
+
V
DROPOUT
DROPOUT
OUT
W
–
. If the external load
OUT
V
19285 F02
OUT
will be reduced
I
OUT
U
IN
A low ESR ceramic capacitor is recommended for the
flying capacitor C
high V
on CPO which would reflect as lower ripple on V
If a minimum enable time is required, the CPO output filter
capacitor should be at least 2 the V
When the LDO is first enabled, the CPO capacitor will
dump a large amount of charge into the V
the drop in the CPO voltage falls below 1.45V
will be disabled and the CPO voltage will be required to
charge up to 1.75V
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 minimize
the spike component, however, low ESR capacitors are
essential to reduce the remaining spike energy effect on
the CPO voltage. C
currents to minimize the droop component. Ripple com-
ponents on CPO are greatly reduced at V
however, C
improve filtering of the CPO noise.
Shutdown
When CN/SHDN = 0V, the part will be in shutdown, the
supply current will be < 8 A and V
ground through a 160 switch. In addition, CPO will be
high impedance and disconnected from V
CN/SHDN.
Shutdown is achieved by internally sampling the
CN/SHDN pin for a low voltage. Time between shutdown
samples is about 30 s. During the sample time the charge
pump switches are disabled and CN/SHDN must be pulled
to ground within 400ns. A resistor value between 100
and 1k is recommended. Parasitic lead capacitance should
be minimized on the CN/SHDN pin.
IN
a smaller capacitor could be used to reduce ripple
OUT
should also be a low ESR capacitor to
FLY
CPO
IN
with a value of 0.47 F. At low load or
to enable the LDO. The resulting
should be increased for high load
OUT
OUT
will be shorted to
OUT
OUT
filter capacitor.
capacitor. If
by the LDO,
IN
the LDO
OUT
IN
.
and