ltc1751 Linear Technology Corporation, ltc1751 Datasheet - Page 7

• Manufacturer Part Number: ltc1751
• Description: Micropower, Regulated Charge Pump Dc/dc Converters
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Operation (Refer to Simplified Block Diagrams)
The LTC1751 family uses a switched capacitor charge
pump to boost V
tion is achieved by sensing the output voltage through a
resistor divider and enabling the charge pump when the
divided output drops below the lower trip point of COMP1.
When the charge pump is enabled, a 2-phase
nonoverlapping clock activates the charge pump switches.
The flying capacitor is charged to V
clock. On phase 2 of the clock, it is stacked in series with
V
discharging the flying capacitor continues at the clock
frequency until the divided output voltage reaches the
upper trip point of COMP1. Once this happens the charge
pump is disabled. When the charge pump is disabled the
device typically draws less than 20 A from V
providing high efficiency under low load conditions.
In shutdown mode all circuitry is turned off and the
LTC1751 draws only leakage current from the V
Furthermore, V
pin is a CMOS input with a threshold voltage of approxi-
mately 0.8V. The LTC1751 is in shutdown when a logic low
is applied to the SHDN pin. The quiescent supply current
of the LTC1751 will be slightly higher if the SHDN pin is
driven high with a voltage that is below V
driven all the way to V
impedance CMOS input it should never be allowed to float.
To ensure that its state is defined it must always be driven
with a valid logic level.
Power Efficiency
The efficiency ( ) of the LTC1751 family is similar to that
of a linear regulator with an effective input voltage of twice
the actual input voltage. This occurs because the input
current for a voltage doubling charge pump is approxi-
mately twice the output current. In an ideal regulated
doubler the power efficiency would be given by:
IN
and connected to V
P
P
OUT
IN
OUT
V
V
IN
OUT
IN
to a regulated output voltage. Regula-
U
is disconnected from V
• 2
•
OUT
I
I
OUT
IN
OUT
. Since the SHDN pin is a high
U
. This sequence of charging and
V
2
OUT
V
IN
W
IN
on phase 1 of the
IN
IN
. The SHDN
than if it is
U
IN
IN
supply.
thus
LTC1751/LTC1751-3.3/LTC1751-5
At moderate to high output power, the switching losses
and quiescent current of the LTC1751 are negligible and
the expression is valid. For example, an LTC1751-5 with
V
measured efficiency of 82% which is in close agreement
with the theoretical 83.3% calculation. The LTC1751 prod-
uct family continues to maintain good efficiency even at
fairly light loads because of its inherently low power
design.
Short-Circuit/Thermal Protection
During short-circuit conditions, the LTC1751 will draw
between 200mA and 400mA from V
junction temperature. On-chip thermal shutdown circuitry
disables the charge pump once the junction temperature
exceeds approximately 160 C and re-enables the charge
pump once the junction temperature drops back to ap-
proximately 150 C. The device will cycle in and out of
thermal shutdown indefinitely without latchup or damage
until the short circuit on V
V
The style and value of capacitors used with the LTC1751
family determine several important parameters such as
output ripple, charge pump strength and minimum
start-up time.
To reduce noise and ripple, it is recommended that low
ESR (< 0.1 ) capacitors be used for both C
These capacitors should be either ceramic or tantalum and
should be 6.8 F or greater. Aluminum capacitors are not
recommended because of their high ESR. If the source
impedance to V
may not be needed. Alternatively, a somewhat smaller
value of input capacitor may be adequate, but will not be
as effective in preventing ripple on the V
The value of C
Increasing the size of C
the output ripple at the expense of higher minimum turn on
time and higher start-up current. See the section Output
Ripple.
IN
IN
, V
= 3V, I
OUT
Capacitor Selection
OUT
OUT
= 50mA and V
IN
is very low, up to several megahertz, C
controls the amount of output ripple.
OUT
OUT
to 10 F or greater will reduce
OUT
is removed.
regulating to 5V, has a
IN
causing a rise in the
IN
pin.
IN
and C
OUT
7
IN
.