ISL6261 Datasheet, PDF (19/34 Pages) Intersil Corporation – Single-Phase Core Regulator for IMVP-6 Mobile CPUs

ISL6261
OC
Internal to
ISL6261
1
10uA
OCSET
VSUM
DROOP
DFB
DROOP
VO
R_ocset
VO
R_s
R_par
R_series
V_dcr I_o DCR
R_ntc
Rn
(R_ntc+R_series) R_par
R_ntc+R_series+R_par
FIGURE 9. EQUIVALENT MODEL FOR DROOP CIRCUIT USING DCR SENSING
Static Mode of Operation - Static Droop Using DCR
Sensing
The ISL6261 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 9 shows the
simplified model of the droop circuitry. The inductor DC
current generates a DC voltage drop on the inductor DCR.
Equation 15 gives this relationship.
V_DCR = I _o × DCR
(EQ. 15)
An R-C network senses the voltage across the inductor to
get the inductor current information. R_n represents the NTC
network consisting of R_ntc, R_series and R_par. The choice of R_s
will be discussed in the next section.
The first step in droop load line compensation is to choose
R_n and R_s such that the correct droop voltage appears even
at light loads between the VSUM and VO nodes. As a rule of
thumb, the voltage drop across the R_n network, V_n, is set to
be 0.5-0.8 times V_DCR. This gain, defined as G1, provides a
fairly reasonable amount of light load signal from which to
derive the droop voltage.
The NTC network resistor value is dependent on the
temperature and is given by:
R_n
(T )
=
(R_series + R_ntc ) ⋅ R_par
R_series + R_ntc + R_par
(EQ. 16)
G1, the gain of V_n to V_DCR, is also dependent on the
temperature of the NTC thermistor:
G₁
(T
)
Δ
=
R_n
R_n
(T
(T )
) + R_s
(EQ. 17)
The inductor DCR is a function of the temperature and is
approximately given by
DCR(T ) = DCR_25C ⋅ (1 + 0.00393* (T − 25)) ^{(EQ. 18)}
in which 0.00393 is the temperature coefficient of the copper.
The droop amplifier output voltage divided by the total load
current is given by:
R_droop = G₁(T)⋅ DCR(T ) ⋅ k_droopamp
(EQ. 19)
R_droop is the actual load line slope. To make R_droop
independent of the inductor temperature, it is desired to
have:
G₁ (T ) ⋅ (1 + 0.00393* (T − 25)) ≅ G_{1t arget}
(EQ. 20)
where G_1target is the desired ratio of V_n/V_DCR. Therefore, the
temperature characteristics G₁ is described by:
G₁ (T)
=
(1+
G_{1t arg et}
0.00393* (T
−
25)
(EQ. 21)
For different G1 and NTC thermistor preference, Intersil
provides a design spreadsheet to generate the proper value
of R_ntc, R_series, R_par.
19
FN9251.1
September 27, 2006