LTC3577 datasheet(32/52 Pages) LINER

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LTC3577/LTC3577-1

3577f

OPERATION

increases until it is turned on continuously at 100%. In this

dropout condition, the respective output voltage equals the

regulator’s input voltage minus the voltage drops across

the internal P-channel MOSFET and the inductor.

Soft-Start Operation

Soft-start is accomplished by gradually increasing the peak

inductor current for each step-down switching regulator

over a 500μs period. This allows each output to rise slowly,

helping minimize inrush current required to charge up the

switching regulator output capacitor. A soft-start cycle

occurs whenever a given switching regulator is enabled.

A soft-start cycle is not triggered by changing operating

modes. This allows seamless output transition when

actively changing between operating modes.

Slew Rate Control

The step-down switching regulators contain new patent

pending circuitry to limit the slew rate of the switch node

(SW1, SW2 and SW3). This new circuitry is designed to

transition the switch node over a period of a few nanosec-

onds, signiﬁcantly reducing radiated EMI and conducted

supply noise while maintaining high efﬁciency. Since

slowing the slew rate of the switch nodes causes efﬁciency

loss, the slew rate of the step-down switching regulators is

adjustable via the I2C registers SLEWCTL1 and SLEWCTL2.

This allows the user to optimize efﬁciency or EMI as neces-

sary with four different slew rate settings. The power-up

default is the fastest slew rate (highest efﬁciency) setting.

Figures 14 and 15 show the efﬁciency and power loss

graph for Buck3 programmed for 1.2V and 2.5V outputs.

Note that the power loss curves remain fairly constant for

both graphs yet changing the slew rate has a larger effect

on the 1.2V output efﬁciency. This is mainly because for

a given output current the 2.5V output is delivering more

than 2x the power than the 1.2V output. Efﬁciency will

always decrease and show more variation to slew rate as

the programmed output voltage is decreased.

Low Supply Operation

An undervoltage lockout circuit on VOUT (VOUT UVLO)

shuts down the step-down switching regulators when VOUT

drops below about 2.7V. It is recommended that the step-

down switching regulator input supplies (VIN12, VIN3) be

connected to the power path output (VOUT) directly. This

UVLO prevents the step-down switching regulators from

operating at low supply voltages where loss of regula-

tion or other undesirable operation may occur. If driving

the step-down switching regulator input supplies from

a voltage other than the VOUT pin, the regulators should

not be operated outside the speciﬁed operating range as

operation is not guaranteed beyond this range.

Inductor Selection

Many different sizes and shapes of inductors are available

from numerous manufacturers. Choosing the right inductor

from such a large selection of devices can be overwhelming,

but following a few basic guidelines will make the selection

Figure 14. VOUT3 (1.2V) Efﬁciency and Power Loss vs IOUT3

Figure 15. VOUT3 (2.5V) Efﬁciency and Power Loss vs IOUT3

IOUT3 (mA)

1.00E-05

100

1.00E-01

3577 F14

1.00E-02

1.00e-01

1.00E+00

1.00E-03

1.00E-04

1.00E-05

1.00E-0.3

Burst Mode

OPERATION

VIN = 3.8V

SW[1:0] =

IOUT3 (mA)

1.00E-05

100

1.00E-01

3577 F15

1.00E-02

1.00e-01

1.00E+00

1.00E-03

1.00E-04

1.00E-05

1.00E-0.3

Burst Mode

OPERATION

VIN = 3.8V

SW[1:0] =

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