IR36021 datasheet(22/41 Pages) IRF

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IR36021

DIGITAL POL BUCK CONTROLLER with I2C AND PMBus

Interface

February 27, 2013 | Rev 3.5

LOAD LINE

The IR36021 enables the implementation of accurate,

temperature compensated load lines on both loops.

The load line is set by an external resistor RCS, as shown

in Figure 24 and the nominal value must also be stored

in MTP. The stored load line, scaling and gain values

provides the IR36021 with the scaling factor for the digital

computation of the total current to determine the OCP

threshold and I2C current and output voltage reporting.

The load line ranges for IR36021 are shown in Table 7.

TABLE 7: LOAD LINE SETTINGS

Loop #1

Loop #2

Minimum

0.0 mΩ

Maximum

6.375 mΩ

12.75 mΩ

Resolution

0.025 mΩ

0.050 mΩ

Figure 24 shows a typical 1.3mΩ loadline measurement

with minimum and maximum error ranges. The controller

accuracy lies well within common processor requirements.

For each loop, the sensed current from all the active

phases is summed and applied to a resistor network across

the RSCP and RCSM pins. This generates a precise

proportional voltage which is summed with the sensed

output voltage and VID DAC reference to form the error

voltage. Also part of the network shown in Figure 32

is thermistor, RTh. For proper loadline temperature

compensation, the thermistor is placed near the phase one

inductor to accurately sense the inductor temperature.

Figure 24: Load Line & Thermal Compensation

The resistor RCS is calculated using the following procedure:

First the designer calculates the RCSeffective or the total

effective parallel resistance across the RSCP and RCSM

pins. It is defined by:

DCR

ISEN

effective





Where RLL is the desired loadline, typically 1.0mΩ,

DCR is DC resistance of the phase inductor, and R_ISEN

is the series resistor across the inductor sense circuit.

The required value for R_ISEN is a 301Ω, 1% tolerance.

Then the designer chooses a suitable NTC thermistor.

Thermistor Rth is typically selected to have the lowest

thermal coefficient and tightest tolerance in a standard

available package. A typical value for the NTC will be 10kΩ,

1% tolerance. Recommended thermistors are shown in

Table 8.

TABLE 8: 10K 1% NTC THERMISTORS

Murata

NCP18XH103F03RB

Panasonic

ERTJ1VG103FA

TDK

NTCG163JF103F

Then the designer calculates RCS using the following

equation:

series

effective









Rseries is selected to achieve minimum loadline error over

temperature. The IR DPDC (GUI) provides a graphical tool

that allows the user to easily calculate the resistor values

for minimum error.

The capacitor CCS is defined by the following equation:

AVP

RCSeffecti







where, fAVP is the user selectable current sense AVP

bandwidth. The best bandwidth is typically in the range

of 200kHz to 300kHz.

Setting 0mΩ Loadline

The load line is turned off by setting a digital bit in the

IR36021 register map. This is a separate bit from the load

line settings for each loop.

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