IR3531 datasheet(34/39 Pages) IRF

34 / 39 page

4+1 Phase Dual Output Control IC

IR3531

March 22, 2012 | FINAL | V2.27

RAIL1 THERMAL COMPENSATION

RSCALE1, RSCALE2, RSCALE3 and RTHERM3 are used to

provide current reporting thermal compensation for Rail1.

The purpose is to keep VDRP1-VDAC1 equal to 900mV for

all temperatures at the maximum load current. This is

expressed mathematically in the following equation.

where DCR and RTHERM3 are expressed in section Rail0

Thermal Compensation. Imax is the maximum current for

Rail1 and Gcs is the typical current sense amplifier gain of

32.5. The temperature sensing components are chosen by

finding an approximate solution that results in VDRP1-

VDAC1=900mV over the entire temperature operating

range. This can be done using an optimization routine of

your choice such as the IR3531 excel design tool.

RAIL 1 DROOP RESISTOR CALCULATION

RDRP1 in combination with the feedback resistor RFB1

sets the load line of Rail1. RFB1 is first chosen with a

typical suggested value of 2kOhm. The equation below

calculates RDRP1.





















)

(

RSCALE

RTHERM

RSCALE

RTHERM

RSCALE

RTHERM

RSCALE

RTHERM

RSCALE

Gcs

DCR

RFB

RDRP

ROOM

where Ro is the load line, DCRROOM is the inductor series

resistance at room temperature, Gcs is the typical current

sense amplifier gain of 32.5, RTHERM3ROOM value at room

temperature.

COMPENSATION NETWORKS

IR3531 utilizes voltage mode control for small signal loop

regulation. The compensation scheme is a classic type 3

system consisting of components RFB(1), CFB(1), RCFB(1),

CEA(1), CCP(1) and RCP(1).

The system dynamics can change significantly when

transitioning from 4 phases to 1 phase. Loop 0 has an

additional component, RPSC, that is inserted in the loop

when in PS1 mode (single phase) to optimize phase

margin. RPSC adds to RCP thereby reducing the system

bandwidth if desired. To disable this feature, place RPSC

as a zero ohm resistor. The IR3531 excel design tool can

be used to calculate an initial starting point.

Note RDRP needs to be recalculated if RFB is changed.

LAYOUT GUIDELINES



VCC bias inductor LVCC must be close to SW pin.

VCC bias bulk cap COUTVCC must be located near

LVCC and connections for COUTVCC must be as

short as possible.



For both rails, all components connected to EA,

FB, VDRP, and VO pins must be located on the

same layer as the IR3531 as close to these pins as

possible.



Insert 9 equally spaced connection vias to GND

tab of IR3531.



V12V decoupling cap must be near pin of IR3531

with GND connection as short as possible.



ROSC must be located close to pin of IR3531.



RTHERM1 and RTHERM3 must be located close to

inductor of associated voltage regulator. Locate

RTHERM2 to provide overall temperature reading

of the power converter.

RSCALE

RTHERM

RSCALE

RTHERM

RSCALE

RTHERM

RSCALE

RTHERM

RSCALE

Gcs

DCR

VDAC

VDRP

900

)

(

)

(





















IR3531 Datasheet(PDF) 34 Page - International Rectifier

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