ISL6263 Datasheet, PDF (14/19 Pages) Intersil Corporation – 5-Bit VID Single-Phase Voltage Regulator for IMVP-6+ Santa Rosa GPU Core

ISL6263
RBIAS Current Reference
The RBIAS pin is internally connected to a 1.545V reference
through a 3kΩ resistance. A bias current is established by
connecting a ±1% tolerance, 150kΩ resistor between the
RBIAS and VSS pins. This bias current is mirrored, creating the
OCSET reference current I _OCSET that is sourced from the
OCSET pin. Do not connect any other components to this
pin, as they will have a negative impact on the performance
of the IC.
I2UA Current Reference
The I2UA pin is connected to a 2µA current source I_I2UA. This
current source is made available for implementing a voltage
offset of the commanded VID states. A 20kΩ resistor R_I2UA
should be connected across the I2UA and VSS pins if the I_I2UA
current source is unused.
Setting the PWM Switching Frequency
The R³ modulator scheme is not a fixed-frequency
architecture, lacking a fixed-frequency clock signal to
produce PWM. The switching frequency increases during
the application of a load to improve transient performance.
The static PWM frequency varies slightly depending on the
input voltage, output voltage, and output current, but this
variation is normally less than 10% in continuous conduction
mode.
Refer to Figure 2, and find that resistor R_FSET is connected
between the V W and COMP pins. A current is sourced from
VW through R_FSET creating the synthetic ripple window
voltage signal V_W which determines the PWM switching
frequency. The relationship between the resistance of R_FSET
and the switching frequency in CCM is approximately given by
Equation 5:
R_FSET
=
-------------------------¹---------------------------
(T – 0.29×10^–6) ⋅ 47
(EQ. 5)
For example, the value of R_FSET for 300kHz operation is
approximately:
7×10³ = -------------------------------------¹--------------------------------------
(3.33×10^–6 – 0.29×10^–6) ⋅ 47
(EQ. 6)
This relationship only applies to operation in constant
conduction mode because the PWM frequency naturally
decreases as the load decreases while in diode emulation
mode. Note that the Electrical Specifications table gives the
nominal PWM frequency of 333kHz with R_FSET = 7kΩ,
different from the result of equation Equation 6. This is
because the IC is trimmed with V_COMP = 2V which is higher
than the typical value encountered in a typical application.
Static Droop Design Using DCR Sensing
The ISL6263 has an internal differential amplifier to
accurately regulate the voltage at the processor die.
For DCR sensing, the process to compensate the DCR
resistance variation takes several iterative steps. Figure 2
shows the DCR sensing method. Figure 8 shows the
simplified model of the droop circuitry. The inductor DC
current generates a DC voltage drop on the inductor DCR.
Equation 7 gives this relationship:
V_DCR = I_o ⋅ DCR
(EQ. 7)
An R-C network senses the voltage across the inductor to
get the inductor current information. R_NTCEQ represents the
NTC network consisting of R_NTC, R_{NTCS, and} R_NTCP. The
choice of R_S will be discussed in the next section.
The first step in droop load line compensation is to adjust
R_NTCEQ, and R_S such that the correct droop voltage
appears even at light loads between the VSUM and VO pins.
As a rule of thumb, the voltage drop V_N across the R_NTCEQ
network, is set to be 0.5x to 0.8x V_DCR. This gain, defined as
G₁, provides a reasonable amount of light load signal from
which to derive the droop voltage.
The NTC network resistor value is dependent on
temperature and is given by Equation 8:
R_N(T)
=
(_---R_{----N-----T---C------+----}R_{-----N----T----C----S----})_---⋅_---R_{-----N----T----C----P--}
R_NTC + R_NTCS + R_NTCP
(EQ. 8)
G₁, the gain of V_N to V_DCR, is also dependent on the
temperature of the NTC thermistor:
G₁(T)
=
--------^R----^N-----⁽--^T----⁾--------
R_N(T) + R_S
(EQ. 9)
The inductor DCR is a function of temperature and is
approximately given by Equation 10:
DCR(T) = DCR_25°C ⋅ (1 + 0.00393 ⋅ (T – 25°C))
(EQ. 10)
The droop amplifier output voltage divided by the total load
current is given by Equation 11:
R_droop = G₁(T) ⋅ DCR_25°C ⋅ (1 + 0.00393 ⋅ (T – 25°C)) ⋅ k_droopamp
(EQ. 11)
R_droop is the actual load line slope, and 0.00393 is the
temperature coefficient of the copper. To make R_droop
independent of the inductor temperature, it is desired to
have Equation 12:
G₁(T) ⋅ (1 + 0.00393 ⋅ (T – 25°C)) ≅ G_{1t arget}
(EQ. 12)
where G_1target is the desired ratio of V_n / V_DCR. Therefore,
the temperature characteristics G₁ is described by
Equation 13:
G₁(T)
=
------------------------^G-----¹---^t--^a---^r--^g---^e----^t-----------------------
(1 + 0.00393 ⋅ (T – 25°C))
(EQ. 13)
It is recommended to begin your droop design using the
R_NTC, R_NTCS, and R_NTCP component values of the
evaluation board available from Intersil.
14
FN9213.2
June 10, 2010