LTC3203_15 Datasheet, PDF (10/16 Pages) Linear Technology – 500mA Output Current Low Noise Dual Mode Step-Up Charge Pumps

LTC3203/LTC3203-1
LTC3203B/LTC3203B-1
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_1.5XIdeal
=
P_OUT
P_IN
=
V_OUT • I_OUT
V_IN • 1.5I_OUT
=
V_OUT
1.5V_IN
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_2XIdeal
=
P_OUT
P_IN
=
V_OUT • I_OUT
V_IN • 2I_OUT
=
V_OUT
2V_IN
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.
V_OUT ²
LTC3203/
LTC3203B ₅
FB
C_FB R1
C_OUT
9, 11
GND
R2
3203 F01
Figure 1. Programming the LTC3203/LTC3203B Output Voltage
The voltage divider ratio is given by the expression:
R1
R2
=
V_OUT
0.91V
−1
or
V_OUT
=
⎛
⎝⎜
R1
R2
+
1^{⎞⎠⎟}
• 0.91V
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.
2
V_OUT
I_LED =
V_FB
R_X
LTC3203/
LTC3203B
5
FB
•••
GND
9, 11
C_OUT
R_X
R_X
3203 F02
Figure 2. Programming the LTC3203/LTC3203B Output Current
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
32031fa
10