LTC3441 Linear Technology, LTC3441 Datasheet - Page 17

• Manufacturer Part Number: LTC3441
• Description: Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
USB Pin and Wall Adapter Capacitor Selection
Caution must be exercised when using ceramic capacitors
to bypass the USB pin or the wall adapter input. High
voltage transients can be generated when the USB or wall
adapter is hot plugged. When power is supplied via the
USB bus or wall adapter, the cable inductance along with
the self resonant and high Q characteristics of ceramic
capacitors can cause substantial ringing which can easily
exceed the maximum voltage pin ratings and damage the
LTC3455. Refer to Linear Technology Application Note 88,
entitled “Ceramic Input Capacitors Can Cause Overvoltage
Transients” for a detailed discussion of this problem. The
long cable lengths of most wall adapters and USB cables
makes them especially susceptible to this problem. Even
if this ringing is not large enough to damage the part, it can
couple to the V
outputs) and be mistaken as loop instability. To bypass the
USB pin and the wall adapter input, add a 1 resistor in
series with a ceramic capacitor to lower the effective Q of
the network and greatly reduce the ringing. A tantalum,
OS-CON, or electrolytic capacitor can be used in place of
the ceramic and resistor, as their higher ESR reduces the
Q, thus reducing the voltage ringing. Use 4.7 F to 10 F for
the USB pin, and 1 F or larger for the wall adapter input.
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.
V
OUT
0 8
.
Figure 6. Setting the Output Voltage
V
MAX
•
LTC3455
1
U
FB1, FB2
pin (and to the switching regulator
R
R
GND
2
1
U
1, 18
25
W
V
OUT
R1
R2
3455 F06
U
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 7, the
efficiency at low load currents increases significantly
when Burst Mode operation is used.
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 8 shows the switching waveforms for switcher 1
(both PWM mode and Burst Mode Operation) with V
3.6V, V
Burst Mode is a registered trademark of Linear Technology Corporation.
OUT1
100
Figure 7. PWM and Burst Mode Efficiency
90
80
70
60
50
40
30
20
= 1.8V, and I
1
Burst Mode
3.3V
Mode
Burst
1.8V
PWM Mode
1.8V
MAX
LOAD CURRENT (mA)
10
PWM Mode
to always allow automatic Burst
OUT1
3.3V
= 25mA.
100
V
BAT
= 3.6V
LTC3455
3455 F07
1000
17
IN
3455f
=