IR3086AM datasheet(16/33 Pages) IRF

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IR3086A

IR3086A EXTERNAL COMPONENTS

PWM Ramp Resistor RPWMRMP and Capacitor CPWMRMP

PWM ramp is generated by connecting the resistor RPWMRMP between a voltage source and PWMRMP pin as well

as the capacitor CPWMRMP between PWMRMP and LGND. Choose the desired PWM ramp magnitude VRAMP and

the capacitor CPWMRMP in the range of 100pF and 470pF, and then calculate the resistor RPWMRMP from Equation

(15). To achieve feed-forward voltage mode control, the resistor RRAMP should be connected to the input of the

converter.

)]

ln(

)

[ln(

PWMRMP

DAC

PWMRMP

−

(15)

Inductor Current Sensing Capacitor CCS+ and Resistors RCS+ and RCS-

The DC resistance of the inductor is utilized to sense the inductor current. Usually the resistor RCS+ and capacitor

CCS+ in parallel with the inductor are chosen to match the time constant of the inductor, and therefore the voltage

across the capacitor CCS+ represents the inductor current. If the two time constants are not the same, the AC

component of the capacitor voltage is different from that of the real inductor current. The time constant mismatch

does not affect the average current sharing among the multiple phases, but affect the current signal ISHARE as well

as the output voltage during the load current transient if adaptive voltage positioning is adopted.

Measure the inductance L and the inductor DC resistance RL. Pre-select the capacitor CCS+ and calculate RCS+ as

follows.

+ =

(16)

The bias current flowing out of the non-inverting input of the current sense amplifier creates a voltage drop across

RCS+, which is equivalent to an input offset voltage of the current sense amplifier. The offset affects the accuracy of

converter current signal ISHARE as well as the accuracy of the converter output voltage if adaptive voltage

positioning is adopted. To reduce the offset voltage, a resistor RCS- should be added between the amplifier inverting

input and the converter output. The resistor RCS- is determined by the ratio of the bias current from the non-inverting

input and the bias current from the inverting input.

−

∗

CSIN

(17)

If RCS- is not used, RCS+ should be chosen so that the offset voltage is small enough. Usually RCS+ should be less

than 2 kΩ and therefore a larger CCS+ value is needed.

Over Temperature Setting Resistors RHOTSET1 and RHOTSET2

The threshold voltage of VRHOT comparator is proportional to the die temperature TJ (ºC) of phase IC. Determine

the relationship between the die temperature of phase IC and the temperature of the power converter according to

the power loss, PCB layout and airflow etc, and then calculate HOTSET threshold voltage corresponding to the

allowed maximum temperature from Equation (18).

241

−

HOTSET

(18)

There are two ways to set the over temperature threshold, central setting and local setting. In the central setting,

only one resistor divider is used, and the setting voltage is connected to HOTSET pins of all the phase ICs. To

reduce the influence of noise on the accuracy of over temperature setting, a 0.1uF capacitor should be placed next

to HOTSET pin of each phase IC. In the local setting, a resistor divider per phase is needed, and the setting voltage

is connected to HOTSET pin of each phase. The 0.1uF decoupling capacitor is not necessary. Use VBIAS as the

reference voltage. If RHOTSET1 is pre-selected, RHOTSET2 can be calculated as follows.

HOTSET

BIAS

HOTSET

−

∗

(19)

Page 16 of 33

1/31/05

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