IR3521 datasheet(29/45 Pages) IRF

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IR3521

Page 29

V3.03

DOWN

SINK

VDAC



(5)

VDAC









(6)

Over Current Setting Resistor ROCSET

The inductor DC resistance is utilized to sense the inductor current. The copper wire of inductor has a constant

temperature coefficient of 3850 ppm/°C, and therefore the maximum inductor DCR can be calculated from (7), where

RL_MAX and RL_ROOM are the inductor DCR at maximum temperature, TL_MAX, and room temperature, TL_ROOM,

respectively.

)]

(

3850

[

ROOM

MAX

ROOM

MAX













(7)

The total input offset voltage (VCS_TOFST), of the phase IC’s current sense amplifier, is the sum of input offset

(VCS_OFST) of the amplifier itself and that created by the amplifier input bias current flowing through the current sense

resistor RCS.

CSIN

OFST

TOFST









(8)

The over-current limit is set by the external resistor ROCSET as defined in (9). ILIMIT is the required over-current limit.

IOCSET is the bias current of OCSET pin and can be calculated with the equation located in the ELECTRICAL

CHARACTERISTICS Table. GCS is the gain of the current sense amplifier. KP is the ratio of inductor peak current

over the average current in each phase and can be calculated using equation (10).

OCSET

TOFST

MAX

LIMIT

OCSET

]

)

(

[









(9)

)

(







(10)

VCCL Programming Resistor RVCCLFB1 and RVCCLFB2

Since VCCL voltage is proportional to the MOSFET gate driver loss and inversely proportional to the MOSFET

conduction loss, the optimum voltage should be chosen to maximize the converter efficiency. VCCL linear

regulator consists of an external NPN transistor, a ceramic capacitor and a programmable resistor divider. Pre-

select RVCCLFB1, and calculate RVCCLFB2 from (11).





VCCL

VCCLFB

(11)

No Load Offset Setting Resistor RFB11, RFB13, RTHERM1 and Adaptive Voltage Positioning Resistor

RDRP11

for Output1

Define RFB_R as the effective offset resistor at room temperature equals to RFB11//(RFB13+RTHERM1). Given the

offset voltage VO_NLOFST (offset above the DAC voltage) and calculating the sink current from the FB1 pin IFB1, using

the equation in the ELECTRICAL CHARACTERISTICS Table, the effective offset resistor value, RFB1, can be

determined from equation (12).

IR3521 Datasheet(PDF) 29 Page - International Rectifier

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