ISL6263C Datasheet, PDF (14/18 Pages) Intersil Corporation – 5-Bit VID Single-Phase Voltage Regulator with Current Monitor for GPU Core Power

ISL6263C
VDD
−
OCP
+
+
−
10µA _↓
OCSET
+
I_SENSE
−
ISP
ISN
ICOMP
VO
R_OCSET
R_S
+
V_N
V_DCR
-
FIGURE 8. EQUIVALENT MODEL OF CURRENT SENSE USING INDUCTOR DCR CURRENT SENSE
and the switching frequency in CCM is approximated using
Equation 6:
R_FSET
=
-⁽--^t---–-----⁰---^.--⁵----^×-----¹----⁰---^–---⁶----⁾
400 × 10^–12
(EQ. 6)
t is the switching period. For example, the value of R_FSET for
300kHz operation is approximated using Equation 7:
7.1 ×10³
=
-⁽--³---^.--³---³-----^×-----¹---⁰----^–---⁶----–-----⁰---^.--⁵----^×-----¹----⁰---^–---⁶----⁾
400 × 10^–12
(EQ. 7)
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.
Inductor DCR Current Sense
ISL6263C provides the option of using the inductor DCR for
current sense. To maintain the current sense accuracy, an
NTC compensation network is optional when using DCR
sense. The process to compensate the DCR resistance
variation takes several iterative steps. Figure 2 shows the
DCR sense method. Figure 8 shows the simplified model of
the current sense circuitry. The inductor DC current I_O
generates a DC voltage drop on the inductor DCR.
Equation 8 gives this relationship:
V_DCR = I_O ⋅ DCR
(EQ. 8)
An R-C network senses the voltage across the inductor to
get the inductor current information. R_N represents the
equivalent resistance of R_P and the optional NTC network
consisting of R_NTC and R_NTCS. R_N is temperature T
dependent and is given by Equation 9:
R_N(T)
=
(_---R_{----N-----T---C------+-----}R_{----N----T----C----S----})_---⋅_----R_----P--
R_NTC + R_NTCS + R_P
(EQ. 9)
If the NTC network is not used, simply set R_N(T) = R_P.
Sensing the time varying inductor current accurately
requires that the parallel R-C network time constant match
the inductor L/DCR time constant. Equation 10 shows this
relationship:
------^L-------
DCR
=
⎛
⎜
⎝
R^-R----_N^-N---(^-(--T^-T---)^{-)--+-⋅---R-}R^----S-_S^--_{⎠⎟⎞}
⋅ C_N
(EQ. 10)
Solution of C_N yields:
⎛
⎝
D^-----C^-L----R^{---⎠⎞}
C_N
=
------------------------------------
⎛
⎜
⎝
R^-R----_N^-N---(^-(--T^-T---)^{-)--+-⋅---R-}R^----S-_S^--_{⎠⎟⎞}
(EQ. 11)
The first step is to adjust R_N(T) and R_S such that the correct
current information appears between the ISP and VO pins
even at light loads. Assume V_N is the voltage drop across
R_N(T). The V_N to V_DCR gain G₁(T) provides a reasonable
amount of light load signal from which to derive the current
information. G₁(T) is given by Equation 12:
G₁(T) = -_R----_N---^R-₍---_T^N----₎-⁽--^T_+----⁾-_R-----_S--
(EQ. 12)
The gain of the current sense amplifier circuit is expressed in
Equation 13:
K_ISENSE
=
1
+
R_{-----I--S----2-}
R_IS1
(EQ. 13)
The current sense amplifier output voltage is given by
Equation 14:
V_ICOMP = V_O + V_N ⋅ K_ISENSE
(EQ. 14)
The inductor DCR is a function of temperature T and is
approximated using Equation 15:
DCR(T) = DCR(+25°C) ⋅ (1 + 0.00393 ⋅ T – (+25°C))
(EQ. 15)
14
FN6745.1
July 8, 2010