LTC3589 datasheet(42/50 Pages) LINER

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LTC3589/LTC3589-1/

LTC3589-2

3589fe

LDO2 and step-down switching regulators 1 to 3 each have

a pair of control bits in the voltage change control register

VCCR. The reference select bit selects which of two 5-bit

words are used as inputs to the regulators feedback refer-

ence DAC inputs. The slew GO bit initiates a DAC slew to

the voltage selected by the reference select bit. When the

slew is complete, the slew GO bits are reset LOW.

Accessing the CLIRQ command register will clear the IRQ

pin and will let the IRQ pin to release HIGH. The pin is

cleared when the LTC3589 acknowledges the sub address.

Data written to the CLIRQ command register is ignored.

There are eight command registers that are used to store

the 5-bit dynamic target voltage input to the feedback

reference slewing DACs – B1DTV1, B1DTV2, B2DTV1,

B2DTV2, B3DTV1, B3DTV2, L2DTV1 and L2DTV2. The

registers ending with V2 use bits 4 through 0 to store

the V2 feedback reference voltage for the regulators. The

regulators input reference voltage is set to V2 by setting

the reference select bits HIGH in VCCR and writing to the

go bits in VCCR. The V2 voltage is also selected whenever

the VSTB pin is driven HIGH. The registers ending with

V1 use bits 4 through 0 to store the V1 feedback voltage

reference for the regulators. The regulators input refer-

ence voltage is set to V1 voltage by setting the reference

select bits LOW in command register VCCR. Whenever

a new dynamic target voltage is set, either by changing

the 5-bit value or by changing the reference select bits in

VCCR, the go bits in VCCR must be written to initiate the

dynamic voltage slew. When bit 5 in B1DTV1, B2DTV1,

B3DTV1, and L2DTV1 is LOW the PGOOD pin pulls LOW

during a dynamic voltage slew. Bits 7 and 6 in B1DTV1

set the switch DV/DT rate for all the step-down switch-

ing regulators. Bit 5 in registers B1DTV2, B2DTV2 and

B3DTV2 selects the switching frequency of step-down

switching regulators 1, 2 and 3. Writing the bit LOW sets

the switching frequency to 2.25MHz. Writing the bit HIGH

sets the switching frequency to 1.125MHz.

The dynamic slew rates of the four feedback reference

DACs are independently set using bits in voltage ramp

rate command register (VRRCR). The rate shown is the

slew of the DAC output as it slews up or down to its target

value. The slew rate of the output voltage is scaled by the

gain of the resistor divider network that sets the regulator

output voltage. For example, a regulator set to an output

voltage of 1.2V when the dynamic target voltage reference

is 0.75V has a gain of 1.6. Slewing the regulator output

from 1.2V to 1V requires slewing the DAC output down

125mV from 750mV to 625mV. With a VRRCR slew rate

setting of 01 the slew time of the regulator output is 71μs.

THERMAL CONSIDERATIONS AND BOARD LAYOUT

Printed Circuit Board Power Dissipation

In order to ensure optimal performance and the ability

to deliver maximum output power to any regulator, it is

critical that the exposed ground pad on the backside of

the LTC3589 package be soldered to a ground plane on

the board. The exposed pad is the only GND connection

for the LTC3589. Correctly soldered to a 2500mm2 ground

plane on a double sided 1oz copper board the LTC3589

has a thermal resistance (

θJA) of approximately 34°C/W.

Failure to make good thermal contact between the exposed

pad on the backside of the package and an adequately

sized ground plane will result in thermal resistances far

greater than 34°C/W.

To ensure the junction temperature of the LTC3589 die

does not exceed the maximum rated limit and to prevent

overtemperature faults, the power output of the LTC3589

must be managed by the application. The total power

dissipation in the LTC3589 is approximated by summing

the power dissipation in each of the switching regulators

and the LDO regulators.

The power dissipation in a switching regulator is esti-

mated by:

PD(SWX) =(VOUTX •IOUTX)•

100 – Eff

100

Where VOUTX is the programmed output voltage, IOUTX

is the load current and Eff is the % efficiency that can

be measured or looked up in an efficiency table for the

programmed output voltage.

OPERATION

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