LTC3577-4_15 Datasheet, PDF(30/52 Page) Linear Technology – Highly Integrated Portable Product PMIC

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LTC3577-4_15 Datasheet, PDF (30/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

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LTC3577-3/LTC3577-4

OPERATION

Output Voltage Programming

Figure 8 shows the step-down switching regulator ap-

plication circuit. The full-scale output voltage for each

step-down switching regulator is programmed using a

resistor divider from the step-down switching regulator

output connected to the feedback pins (FB1, FB2 and

FB3) such that:

VOUTx

0.8V

â¢

ââ

1ââ â

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.

VIN

MODE PWM

SLEW CONTROL

GND

SWx

CFB

FBx

0.8V

VOUTx

COUT

357734 F08

Figure 8. Step-Down Switching Regulator Application Circuit

Operating Modes

The step-down switching regulators include two possible

operating modes to meet the noise/power needs of a variety

of applications. In pulse-skipping 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 step-down

switching regulator requiring only a single ceramic output

capacitor for stability. At light loads in pulse-skipping 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 (SW1, SW2 or SW3) goes

high impedance and the switch node voltage will ring. This

is discontinuous operation, and is normal behavior for a

switching regulator. At very light loads in pulse-skipping

mode, the step-down switching regulators will automati-

cally skip pulses as needed to maintain output regulation.

At high duty cycle (VOUTX approaching VINX) it is possible

for the inductor current to reverse at light loads causing

the stepped down switching regulator to operate continu-

ously. When operating continuously, regulation and low

noise output voltage are maintained, but input operating

current will increase to a few milliamps.

In Burst Mode operation, the step-down switching regula-

tors automatically switch between ï¬xed frequency PWM

operation and hysteretic control as a function of the load

current. At light loads the step-down switching regulators

control the inductor current directly and use a hysteretic

control loop to minimize both noise and switching losses.

While operating in Burst Mode operation, the output

capacitor is charged to a voltage slightly higher than the

regulation point. The step-down switching regulator then

goes into sleep mode, during which the output capacitor

provides the load current. In sleep mode, most of the

switching regulatorâs circuitry is powered down, helping

conserve battery power. When the output voltage drops

below a pre-determined value, the step-down switching

regulator circuitry is powered on and another burst cycle

begins. The sleep time decreases as the load current

increases. Beyond a certain load current point (about

1/4 rated output load current) the step-down switching

regulators will switch to a low noise constant frequency

PWM mode of operation, much the same as pulse-skip-

ping operation at high loads.

For applications that can tolerate some output ripple

at low output currents, Burst Mode operation provides

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