Datasheet

LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
10
32031fa
APPLICATIO S I FOR ATIO
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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
V
IN
, 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 V
MODEL
,
it is not possible to achieve output voltages greater than
twice the available input voltage in this case. Similarly,
when MODE is greater than V
MODEH
, 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
2091
1
1
2
1091= − =+
⎛
⎝
⎜
⎞
⎠
⎟
.
•.
Typical values for total voltage divider resistance can
range from several kΩs up to 1MΩ. The compensation
capacitor (C
FB
) 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, C
FB
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
V
OUT
FB
GND
C
OUT
• • •
I
LED
=
V
FB
R
X
9, 11
2
5
R
X
R
X
Figure 2. Programming the LTC3203/LTC3203B Output Current
3203 F01
LTC3203/
LTC3203B
C
OUT
V
OUT
FB
GND
R1
R2
2
5
9, 11
C
FB
In this configuration the feedback factor (∆V
OUT
/∆I
OUT
)
will be very near unity since the small signal LED imped-
ance will be considerably less than the current setting