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LTC3203B-1 Datasheet(PDF) 10 Page - Linear Technology

Part No. LTC3203B-1
Description  500mA Output Current Low Noise Dual Mode Step-Up Charge Pumps
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LTC3203B-1 Datasheet(HTML) 10 Page - Linear Technology

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LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
10
32031fa
APPLICATIO S I FOR ATIO
Power Efficiency
The power efficiency (
η) of the LTC3203/LTC3203-1/
LTC3203B/LTC3203B-1 in 1.5x mode is similar to that of
a linear regulator with an effective input voltage of 1.5
times the actual input voltage. This occurs because the
input current for a 1.5x fractional charge pump is approxi-
mately 1.5 times the load current. In an ideal regulating
1.5x charge pump the power efficiency would be given by:
η
15
15
15
.
..
XIdeal
OUT
IN
OUT
OUT
IN
OUT
OUT
IN
P
P
VI
VI
V
V
==
=
Similarly, in 2x mode, the efficiency is similar to that of a
linear regulator with an effective input voltage of twice the
actual input voltage. In an ideal regulating voltage doubler
the power efficiency would be given by:
η
2
22
XIdeal
OUT
IN
OUT
OUT
IN
OUT
OUT
IN
P
P
VI
VI
V
V
==
=
At moderate to high output power the switching losses
and quiescent current of the LTC3203/LTC3203-1/
LTC3203B/LTC3203B-1 are negligible and the expression
above is valid.
As evident from the above two equations, with the same
VIN, the 1.5x mode will give higher efficiency than the
2x mode.
Programming the LTC3203/LTC3203B Output Voltage
(FB Pin)
While the LTC3203-1/LTC3203B-1 have internal resistive
dividers to program the output voltage, the programmable
LTC3203/LTC3203B may be set to an arbitrary voltage via
an external resistive divider. Since it operates as a voltage
doubling charge pump when MODE is less than VMODEL,
it is not possible to achieve output voltages greater than
twice the available input voltage in this case. Similarly,
when MODE is greater than VMODEH, the achievable output
voltage is less than 1.5 times the available input voltage.
Figure 1 shows the required voltage divider connection.
Figure 1. Programming the LTC3203/LTC3203B Output Voltage
The voltage divider ratio is given by the expression:
R
R
V
V
or V
R
R
V
OUT
OUT
1
20 91
1
1
2
10 91
=
=+
⎝⎜
⎠⎟
.
•.
Typical values for total voltage divider resistance can
range from several k
Ωs up to 1MΩ. The compensation
capacitor (CFB) is necessary to counteract the pole caused
by the large valued resistors R1 and R2, and the input
capacitance of the FB pin. For best results, CFB should be
5pF for all R1 or R2 greater than 10k and can be omitted
if both R1 and R2 are less than 10k.
The LTC3203/LTC3203B can also be configured to control
a current. In white LED applications the LED current is
programmed by the ratio of the feedback set point voltage
and a sense resistor as shown in Figure 2. The current of
the remaining LEDs is controlled by virtue of their similar-
ity to the reference LED and the ballast voltage across the
sense resistor.
3203 F02
LTC3203/
LTC3203B
VOUT
FB
GND
COUT
• • •
ILED =
VFB
RX
9, 11
2
5
RX
RX
Figure 2. Programming the LTC3203/LTC3203B Output Current
3203 F01
LTC3203/
LTC3203B
COUT
VOUT
FB
GND
R1
R2
2
5
9, 11
CFB
In this configuration the feedback factor (
∆VOUT/∆IOUT)
will be very near unity since the small signal LED imped-
ance will be considerably less than the current setting


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