LTC3455(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3455
(Rev.:RevA)

Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

Linear Technology 

LTC3455

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):

VOUT = 0.8V • ⎛⎝⎜1+ RR21⎞⎠⎟

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

1, 18

FB1, FB2

LTC3455

GND 25

VOUT

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.

Burst Mode is a registered trademark of Linear Technology Corporation.

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

VBAT = 3.6V

1

10

100

1000

LOAD CURRENT (mA)

3455 F08

Figure 8. 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 9 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

VSW1

2V/DIV

VOUT1

50mV/DIV

AC COUPLED

IL1

100mA/DIV

5µs/DIV

PWM Mode

3455 F09a

VSW1

2V/DIV

VOUT1

10mV/DIV

AC COUPLED

IL1

100mA/DIV

1µs/DIV

3455 F09b

Figure 9. Burst Mode and PWM Mode Waveforms

3455fa

19

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