Datasheet
LTC3455/LTC3455-1
19
3455fc
APPLICATIONS INFORMATION
Programming Switching Regulator Output Voltage
The output voltage for each switching regulator is pro-
grammed using a resistor divider from the output connected
to the feedback pins (FB1 and FB2):
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
0 8 1
2
1
.•
Typical values for R1 are in the range of 80k to 400k.
Figure 7. Setting the Output Voltage
Figure 8. PWM and Burst Mode Effi ciency
Burst Mode Operation
For highest effi ciencies at light loads, both DC/DC convert-
ers are capable of operating in Burst Mode. In this mode,
energy is delivered to the outputs in shorts bursts, which
minimizes switching losses and quiescent-current losses.
Output voltage ripple is slightly higher in this mode, but
effi ciency is greatly improved. As shown in Figure 8, the
effi ciency at low load currents increases signifi cantly when
Burst Mode operation is used.
Tie the MODE pin to V
MAX
to always allow automatic Burst
Mode operation. Even when the MODE pin is high, the
LTC3455/LTC3455-1 will only enter Burst Mode when the
load current is low. For many noise-sensitive systems, Burst
Mode operation might be undesirable at certain times (i.e.
during a transmit or receive cycle of a wireless device),
but highly desirable at others (i.e. when the device is in
low-power standby mode). The MODE pin can be used to
enable or disable Burst Mode operation at any time, offering
both low-noise and low-power operation when they are
needed the most. Burst Mode is disabled initially at startup
(for the fi rst 200ms) and also whenever external power is
available, even if the MODE pin is pulled high.
Figure 9 shows the switching waveforms for switcher 1
(both PWM mode and Burst Mode Operation) with V
IN
=
3.6V, V
OUT1
= 1.8V, and I
OUT1
= 25mA.
Figure 9. Burst Mode and PWM Mode Waveforms
FB1, FB2
GND
LTC3455/
LTC3455-1
V
OUT
R2
R1
1, 18
25
3455 F07
LOAD CURRENT (mA)
1
EFFICIENCY (%)
100
90
80
70
60
50
40
30
20
10 100 1000
3455 F08
V
BAT
= 3.6V
1.8V
Burst
Mode
Burst Mode
3.3V
PWM Mode
1.8V
PWM Mode
3.3V
5μs/DIV
V
SW1
2V/DIV
V
OUT1
50mV/DIV
AC-COUPLED
I
L1
100mA/DIV
3455 F09a
1μs/DIV
V
SW1
2V/DIV
V
OUT1
10mV/DIV
AC-COUPLED
I
L1
100mA/DIV
3455 F09b
PWM Mode
Burst Mode