ltc1754-5 Linear Technology Corporation, ltc1754-5 Datasheet - Page 6

• Manufacturer Part Number: ltc1754-5
• Description: Micropower, Regulated 3.3v/5v Charge Pump With Shutdown In Sot-23
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1754-3.3/LTC1754-5
Operation (Refer To Block Diagram)
The LTC1754 uses a switched-capacitor charge pump to
boost V
achieved by sensing the output voltage through an internal
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 two-phase
nonoverlapping clock activates the charge pump switches.
The flying capacitor is charged to V
clock. On phase two of the clock it is stacked in series with
V
discharging the flying capacitor continues at a free run-
ning frequency of 600kHz (typ). Once the attenuated
output voltage reaches the upper trip point of COMP1, the
charge pump is disabled. When the charge pump is
disabled the LTC1754 draws only 13 A from V
providing high efficiency under low load conditions.
In shutdown mode all circuitry is turned off and the
LTC1754 draws only leakage current from the V
Furthermore, V
pin is a CMOS input with a threshold voltage of approxi-
mately 0.8V, but may be driven to a logic level that exceeds
V
applied to the SHDN pin. Since the SHDN pin is a high
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 LTC1754 is similar to that of a
linear regulator with an effective input voltage of twice the
actual input voltage. This results because the input current
for a voltage doubling charge pump is approximately twice
the output current. In an ideal voltage doubling regulator
the power efficiency would be given by:
At moderate-to-high output power, the switching losses and
quiescent current of the LTC1754 are negligible and the
expression above is valid. For example, an LTC1754-5 with
6
IN
IN
. The LTC1754 is in shutdown when a logic low is
and connected to V
P
IN
P
OUT
IN
to a regulated output voltage. Regulation is
OUT
V
V
OUT
IN
U
is disconnected from V
2
OUT
I
I
OUT
OUT
U
. This sequence of charging and
V
2
OUT
V
W
IN
IN
on phase one of the
IN
. The SHDN
U
IN
supply.
IN
thus
V
measured efficiency of 82.7%, which is in close agreement
with the theoretical 83.3% calculation. The LTC1754 con-
tinues 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 LTC1754 will draw
between 100mA and 400mA from V
junction temperature. On-chip thermal shutdown circuitry
disables the charge pump once the junction temperature
exceeds approximately 150 C and reenables the charge
pump once the junction temperature drops back to ap-
proximately 140 C. The LTC1754 will cycle in and out of
thermal shutdown indefinitely without latchup or damage
until the short circuit on V
Capacitor Selection
The style and value of capacitors used with the
LTC1754 determine several important parameters such as
output ripple, charge pump strength and turn-on 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
be 6.8 F or greater. Aluminum capacitors are not recom-
mended because of their high ESR. If the source imped-
ance to V
not be needed.
A ceramic capacitor is recommended for the flying capaci-
tor with a value in the range of 1 F to 2.2 F. Note that a
large value flying capacitor (> 2.2 F) will increase output
ripple unless C
applications, C
This will reduce output ripple at the expense of maximum
output current and efficiency.
In order to achieve the rated output current it is necessary
to have at least 0.6 F of capacitance for the flying capaci-
tor. Capacitors of different material lose their capacitance
over temperature at different rates. For example, a ceramic
capacitor made of X7R material will retain most of its
capacitance from – 40 C to 85 C, whereas a Z5U or Y5V
style capacitor will lose considerable capacitance over that
IN
= 3V, I
IN
OUT
is very low up to several megahertz, C
FLY
= 25mA and V
OUT
may be reduced to 0.01 F to 0.047 F.
is also increased. For very low load
OUT
OUT
is removed.
regulating to 5V, has a
IN
causing a rise in the
IN
and C
IN
OUT
may
.