LTC3555 Datasheet, PDF(22/32 Page) Linear Technology – High Effi ciency USB Power Manager + Triple Step-Down DC/DC

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LTC3555 Datasheet, PDF (22/32 Pages) Linear Technology – High Effi ciency USB Power Manager + Triple Step-Down DC/DC

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LTC3555/LTC3555-X

OPERATION

Step-Down Switching Regulator Output Voltage

Programming

All three switching regulators can be programmed for

output voltages greater than 0.8V. Switching regulators 2

and 3 have I2C programmable set-points while regulator 1

has a single ï¬xed set-point. The full-scale output voltage for

each switching regulator is programmed using a resistor

divider from the switching regulator output connected to

the feedback pins (FB1, FB2 and FB3) such that:

VOUTX

VFBX

where VFBX ranges from 0.425V to 0.8V for switching

regulators 2 and 3 and VFBX is ï¬xed at 0.8V for switching

regulator 1. See Figure 4

VINx

SWx

LTC3555/

LTC3555-X

CFB

FBx

VOUTx

COUT

GND

3555 F04

Figure 4. Buck Converter Application Circuit

Typical values for R1 are in the range of 40k to 1M. The

capacitor CFB cancels the pole created by feedback resis-

tors and the input capacitance of the FB pin and also helps

to improve transient response for output voltages much

greater than 0.8V. A variety of capacitor sizes can be used

for CFB but a value of 10pF is recommended for most ap-

plications. Experimentation with capacitor sizes between

2pF and 22pF may yield improved transient response.

Step-Down Switching Regulator Operating Modes

The LTC3555 familyâs general purpose switching regula-

tors include four possible operating modes to meet the

noise/power needs of a variety of applications.

In pulse skip mode, an internal latch is set at the start of

every cycle which turns on the main P-channel MOSFET

switch. During each cycle, a current comparator compares

the peak inductor current to the output of an error ampliï¬er.

The output of the current comparator resets the internal

latch which causes the main P-channel MOSFET switch to

turn off and the N-channel MOSFET synchronous rectiï¬er

to turn on. The N-channel MOSFET synchronous rectiï¬er

turns off at the end of the 2.25MHz cycle or if the current

through the N-channel MOSFET synchronous rectiï¬er

drops to zero. Using this method of operation, the error

ampliï¬er adjusts the peak inductor current to deliver the

required output power. All necessary compensation is

internal to the switching regulator requiring only a single

ceramic output capacitor for stability. At light loads in PWM

mode, the inductor current may reach zero on each pulse

which will turn off the N-channel MOSFET synchronous

rectiï¬er. In this case, the switch node (SW) goes high

impedance and the switch node voltage will âringâ. This

is discontinuous mode operation, and is normal behavior

for a switching regulator. At very light loads in pulse skip

mode, the switching regulators will automatically skip

pulses as needed to maintain output regulation.

At high duty cycles (VOUTx > VINx /2) it is possible for the

inductor current to reverse, causing the regulator to operate

continuously at light loads. This is normal and regulation is

maintained, but the supply current will increase to several

milliamperes due to continuous switching.

In forced Burst Mode operation, the switching regulators

use a constant current algorithm to control the inductor

current. By controlling the inductor current directly and

using a hysteretic control loop, both noise and switching

losses are minimized. In this mode output power is limited.

While in forced Burst Mode operation, the output capacitor

is charged to a voltage slightly higher than the regulation

point. The step-down converter then goes into sleep mode,

during which the output capacitor provides the load cur-

rent. In sleep mode, most of the regulatorâs circuitry is

powered down, helping conserve battery power. When the

output voltage drops below a pre-determined value, the

switching regulator circuitry is powered on and another

burst cycle begins. The duration for which the regulator

operates in sleep mode depends on the load current. The

sleep time decreases as the load current increases. The

maximum output current in forced Burst Mode operation is

about 100mA for switching regulators 1 and 2, and about

250mA for switching regulator 3. The step-down switching

regulators will not enter sleep mode if the maximum output

current is exceeded in forced Burst Mode operation and

the output will drop out of regulation. Forced Burst Mode

3555fd

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