LTC3589 datasheet(20/50 Pages) LINER

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LTC3589/LTC3589-1/

LTC3589-2

3589fe

STEP-DOWN SWITCHING REGULATORS

Output Voltage Programming

Each of the step-down converters uses a dynamically

slewing DAC output for its reference. The full-scale output

voltage is set by using a resistor divider connected from

the step-down switching regulator output to the feedback

pins (B1_FB, B2_FB, and B3_FB), as shown in Figure 3.

Set the output voltage of step-down switching regulators

using the following formula:

VOUT = 1+

• (0.3625+BxDTVx • 0.0125)(V)

BxDTVx is the decimal value of the ﬁve bit binary number

in the I2C BxDTV1 or BxDTV2 command registers. BxDTV1

and BxDTV2 default to 11001 to output a reference voltage

of 0.675V. Typical values for R1 are in the range of 40k

to 1M. The capacitor CFB cancels the pole created by the

feedback resistors and the input capacitance on the FB pin

and also helps to improve load step transient response.

A value of 10pF is recommended for most applications.

Experimentation with capacitor sizes between 10pF and

33pF may yield improved transient response.

Operating Modes

The step-down switching regulators include three possible

operating modes to meet the noise and power needs of a

variety of applications.

In pulse-skipping mode, at the start of every cycle, a latch

is set that turns on the main P-channel MOSFET switch.

During the cycle, a current comparator compares the

peak inductor current to the output of an error ampliﬁer.

The output of the current comparator resets the latch.

At this time the P-channel MOSFET switch turns off and

the N-channel MOSFET synchronous rectiﬁer turns on.

The N-channel MOSFET synchronous rectiﬁer will turn

off when the end of the clock cycle is reached or if the

inductor current drops through zero. Using this method

of operation, the error ampliﬁer adjusts the peak inductor

current to deliver the required output power. All necessary

loop 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 that will

turn off the N-channel MOSFET synchronous rectiﬁer. In

this case the switch node (SW1, SW2, or SW3) goes HIGH

impedance and the switch node will ring. This is discon-

tinuous operation and is normal behavior for a switching

regulator. At very light loads in pulse-skipping mode, the

step-down switching regulators will automatically skip

pulses as needed to maintain output regulation. At high

duty cycle (VOUTX > VIN/2) it is possible for the inductor

current to reverse at light loads causing the step-down

switching regulator to operate continuously. When operat-

ing continuously, regulation and low noise output voltage

are maintained, but input operating current will increase

to a few milliamps.

In the forced continuous mode of operation, the inductor

current is allowed to be less than zero over the full range

of duty cycles. Operating in forced continuous mode is

a lower noise option at light loads than pulse-skipping

operation but with the drawback of higher VIN current

OPERATION

Figure 3. Step-Down Switching Regulator

Application Circuit

3589 F03

PVIN

PWM

CONTROL

0.3625V

TO 0.75V

MODE

COUT

CFB

DAC

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