LTC3203 LINER [Linear Technology], LTC3203 Datasheet - Page 10

• Manufacturer Part Number: LTC3203
• Description: 500mA Output Current Low Noise Dual Mode Step-Up Charge Pumps
• Manufacturer: LINER [Linear Technology]

10
APPLICATIO S I FOR ATIO
LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
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:
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:
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
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
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
voltage is less than 1.5 times the available input voltage.
Figure 1 shows the required voltage divider connection.
IN
η
η
, the 1.5x mode will give higher efficiency than the
1 5
2
.
XIdeal
XIdeal
=
=
P
P
OUT
P
IN
P
OUT
IN
U
=
=
V
V
OUT
V
IN
V
IN
OUT
U
•
•
•
2
MODEH
1 5
I
I
OUT
OUT
•
.
I
OUT
I
OUT
W
, the achievable output
=
V
=
2
OUT
V
1 5
IN
V
.
OUT
V
IN
U
MODEL
,
The voltage divider ratio is given by the expression:
Typical values for total voltage divider resistance can
range from several kΩs up to 1MΩ. The compensation
capacitor (C
by the large valued resistors R1 and R2, and the input
capacitance of the FB pin. For best results, C
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.
In this configuration the feedback factor (∆V
will be very near unity since the small signal LED imped-
ance will be considerably less than the current setting
Figure 2. Programming the LTC3203/LTC3203B Output Current
Figure 1. Programming the LTC3203/LTC3203B Output Voltage
R
R
2 0 91
1
=
V
.
OUT
LTC3203B
FB
LTC3203/
V
GND
) is necessary to counteract the pole caused
LTC3203B
LTC3203/
−
V
9, 11
OUT
FB
1
V
or V
GND
2
5
OUT
FB
5
9, 11
2
OUT
C
OUT
I
LED
=
C
FB
⎛
⎝ ⎜
=
R
R
V
R
R
FB
2
1
X
X
R1
R2
+
• • •
1 0 91
⎞
⎠ ⎟
3203 F01
• .
C
OUT
3203 F02
R
X
FB
V
OUT
should be
/∆I
32031fa
OUT
)