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LTC3455

3455f

LOAD CURRENT (mA)

100

1000

3455 F07

VBAT = 3.6V

1.8V

Burst

Mode

Burst Mode

3.3V

PWM Mode

1.8V

PWM Mode

3.3V

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 VMAX 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 1

Ω 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

µF to 10µF for

the USB pin, and 1

µF 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):

OUT =+









.•

Typical values for R1 are in the range of 80k to 400k.

Figure 6. Setting the Output Voltage

FB1, FB2

GND

LTC3455

VOUT

1, 18

3455 F06

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.

Figure 7. PWM and Burst Mode Efficiency

Tie the MODE pin to VMAX 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 VIN =

3.6V, VOUT1 = 1.8V, and IOUT1 = 25mA.

Burst Mode is a registered trademark of Linear Technology Corporation.

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