LTC3455_15 Datasheet, PDF (19/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455/LTC3455-1
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):
V_OUT = 0.8V • ^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
Typical values for R1 are in the range of 80k to 400k.
V_OUT
1, 18
FB1, FB2
LTC3455/
LTC3455-1
GND ²⁵
R2
R1
3455 F07
Figure 7. Setting the Output Voltage
Burst Mode Operation
For highest efficiencies 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
efficiency is greatly improved. As shown in Figure 8, the
efficiency at low load currents increases significantly when
Burst Mode operation is used.
100 _{Burst Mode}
90 ^3.3V
80
70
1.8V
Burst
Mode
60
3.3V
PWM Mode
50
1.8V
PWM Mode
40
30
20
V_BAT = 3.6V
1
10
100
1000
LOAD CURRENT (mA)
3455 F08
Figure 8. PWM and Burst Mode Efficiency
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 first 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.
Burst Mode
V_SW1
2V/DIV
V_OUT1
50mV/DIV
AC-COUPLED
I_L1
100mA/DIV
5μs/DIV
PWM Mode
3455 F09a
V_SW1
2V/DIV
V_OUT1
10mV/DIV
AC-COUPLED
I_L1
100mA/DIV
1μs/DIV
3455 F09b
Figure 9. Burst Mode and PWM Mode Waveforms
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