LTC3589 datasheet(18/44 Pages) LINER

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LTC3589

3589p

LDO Regulator 4

LDO4 has four output voltage options that are controlled

by the contents of command register L2DTV2 bits 6

and 5. By default, pin EN_LDO34 enables and disables

LDO3 and LDO4 simultaneously when command register

bits OVEN[6] and OVEN[7] are LOW. When EN_LDO34 is

LOW, LDO3 and LDO4 are controlled by writing to com-

mand register bits OVEN[6] and OVEN[7] respectively.

When command register bit L2DTV2[7] is HIGH, control of

LDO4 is disconnected from pin EN_LDO34 and controlled

by command register bit OVEN[7] regardless of the status

of EN_LDO34. Table 3 shows the I2C command register

settings used to control LDO4.

Table 3. LDO 4 Command Register Settings

COMMAND

VALUE

SETTING

OVEN[6]

Disable

Enable

SCR2[6]

Wait for Output Below 300mV Before Enable

Enable Immediately

L2DTV2[6-5]

00*

VLDO4 = 2.8V

VLDO4 = 2.5V

VLDO4 = 1.8V

VLDO4 = 3.3V

L2DTV2[7]

LDO4 Enable Controlled by EN_LDO34

LDO4 Enable Controlled by OVEN[6]

* Denotes Default Power-On Value

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

OPERATION

Figure 3. Step-Down Switching Regulator

Application Circuit

3589 F03

PVIN

PWM

CONTROL

0.3625V

TO 0.75V

MODE

COUT

CFB

DAC

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

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