LTC3455 Datasheet, PDF (17/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
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_MAX pin (and to the switching regulator
outputs) and be mistaken as loop instability. To bypass the
USB pin and the wall adapter input, add a 1Ω 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 4.7µF to 10µF for
the USB pin, and 1µF 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):
V_OUT = 0.8V • ^{}1+ ^{RR21}
Typical values for R1 are in the range of 80k to 400k.
1, 18
FB1, FB2
LTC3455
GND ²⁵
V_OUT
R2
R1
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Figure 6. 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 7, 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)
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Figure 7. 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 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_IN =
3.6V, V_OUT1 = 1.8V, and I_OUT1 = 25mA.
Burst Mode is a registered trademark of Linear Technology Corporation.
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