ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 19

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Programming Switching Regulator Output Voltage
The output voltage for each switching regulator is pro-
grammed using a resistor divider from the output con-
nected to the feedback pins (FB1 and FB2):
Typical values for R1 are in the range of 80k to 400k.
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.
Burst Mode is a registered trademark of Linear Technology Corporation.
V
OUT
=
100
Figure 8. PWM and Burst Mode Efficiency
0 8
90
80
70
60
50
40
30
20
.
Figure 7. Setting the Output Voltage
1
Burst Mode
3.3V
Mode
Burst
V
1.8V
•
LTC3455
⎛
⎜
⎝
1
PWM Mode
U
1.8V
FB1, FB2
+
LOAD CURRENT (mA)
R
R
GND
10
2
1
PWM Mode
⎞
⎟
⎠
U
1, 18
25
3.3V
100
V
W
BAT
V
OUT
= 3.6V
R1
R2
3455 F08
3455 F07
1000
U
Tie the MODE pin to V
Mode operation. Even when the MODE pin is high, the
LTC3455 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 avail-
able, 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
3.6V, V
AC COUPLED
AC COUPLED
100mA/DIV
100mA/DIV
50mV/DIV
10mV/DIV
Figure 9. Burst Mode and PWM Mode Waveforms
OUT1
2V/DIV
2V/DIV
V
V
V
V
OUT1
OUT1
SW1
SW1
I
I
L1
L1
= 1.8V, and I
MAX
Burst Mode
PWM Mode
to always allow automatic Burst
OUT1
1µs/DIV
5µs/DIV
= 25mA.
LTC3455
3455 F09b
3455 F09a
19
3455fb
IN
=