Datasheet

13
LTC3251/
LTC3251-1.2/LTC3251-1.5
32511215fb
OPERATIO
U
(Refer to Block Diagram)
At moderate to high output power the switching losses
and quiescent current of the LTC3251 family is negligible
and the expression above is valid. For example with V
IN
=
3.6V, I
OUT
= 200mA and V
OUT
regulating to 1.5V the
measured efficiency is 81% which is in close agreement
with the theoretical 83.3% calculation.
Programming the LTC3251 Output Voltage (FB Pin)
The LTC3251 is programmed to an arbitrary output volt-
age via an external resistive divider. Figure 7 shows the
required voltage divider connection. The voltage divider
ratio is given by the expression:
R
R
V
V
A
B
OUT
=
08
1
.
–
For a 1.5V output, R
O
is 0.085Ω, which produces a 40mV
output change for a 500mA load current step. Thus, the
user may want to target an unloaded output voltage
slightly higher than desired to compensate for the output
load conditions. The output may be programmed for
regulation voltages of 0.9V to 1.6V.
Since the LTC3251 employs a 2-to-1 charge pump archi-
tecture, it is not possible to achieve output voltages
greater than half the available input voltage. The minimum
V
IN
supply required for regulation can be determined by
the following equation:
V
IN(MIN)
≥ 2 • (V
OUT(MIN)
+ I
OUT
• R
OL
)
The compensation capacitor (C
A
) is necessary to counter-
act the pole caused by the large valued resistors R
A
and R
B
,
and the input capacitance of the FB pin. For best results, C
A
should be 5pF for all R
A
or R
B
greater than 10k and can be
omitted if both R
A
and R
B
are less than 10k.
Disabling Spread Spectrum Operation on the
LTC3251-1.2/LTC3251-1.5 (MODE Pin)
Spread spectrum operation can be disabled by driving
MODE high. When Mode is high, switching takes place at
the maximum operating frequency (typ 1.6MHz). The
advantage of spread spectrum operation is that it reduces
the peak noise at and above the operating frequency at the
expense of a slightly increased noise floor and slightly
increased low frequency ripple caused by the converter
compensating for the changing operating frequency. Us-
ers who do not need the peak noise reduction gained by
using spread spectrum may wish to disable spread spec-
trum, thus improving the low frequency input/output
ripple.
Typical values for total voltage divider resistance can
range from several kΩs up to 1MΩ.
The user may want to consider load regulation when
setting the desired output voltage. The closed loop output
impedance of the LTC3251 is approximately:
R
V
V
O
OUT
≅Ω0 045
08
.•
.
Figure 7. Programming the LTC3251
V
OUT
C
OUT
C
A
R
A
R
B
3251 F07
V
OUT
LTC3251
FB
GND
0.8V
R
A
R
B
1 +
()