LTC3444_15 Datasheet, PDF (12/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

LTC3444
U
OPERATIO
Boost Region (V_IN < V_OUT)
Switch A is always on and switch B is always off during this
mode. When the internal control voltage, V_CI, is above
voltage V3, switch pair CD will alternately switch to pro-
vide a boosted output voltage. This operation is typical to
a synchronous boost regulator. The maximum duty cycle
of the converter is limited to 82% typical and is reached
when V_CI is above V4.
CONTROLLING THE OUTPUT VOLTAGE
The output voltage is controlled via a summing resistor
input at the feedback (FB) resistive divider string. Refer to
Figure 1. The output voltage has an inverse relation to the
control voltage as shown in Figure 5. The resistor values
are dependent on the desired output voltage range and the
control voltage range. When using the internal loop com-
pensation, V_C = GND, R1 must be 340k. For external
compensation R1 should be chosen first and R2 and R3
calculated from the following equations.
The resistor values are given by:
R3 = ^{(VCON(MAX) – VCON(MIN))} •R1Ω
V_O(MAX) – V_O(MIN)
R2
=
(V_CON(MAX)
–
1.22
1.22)
–
(1.22
–
V_O(MIN))
Ω
R3
R1
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0.5
1
1.5
2
V_CONTROL
2.5
3444 G01
Figure 5. V_OUT vs V_CONTROL with R1 = 340k, R2 = 249k, and
R3 = 182k, V_CONTROL = 0.5V to 2.5V
3444fb
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