ISL78211 Datasheet, PDF (20/35 Pages) Intersil Corporation – Automotive Single-Phase Core Regulator for IMVP-6™ CPUs

ISL78211
OC
Internal to ISL6261A
1
10µA
OCSET
DROOP
VSUM
DFB
DROOP
VO
Rocset
VO
Rs
Rpar
Rseries
Vdcr ^Io DCR
Rntc
(Rntc +Rseries) Rpar
Rn
Rntc +Rseries +Rpar
FIGURE 9. EQUIVALENT MODEL FOR DROOP CIRCUIT USING DCR SENSING
Static Mode of Operation - Static Droop
Using DCR Sensing
The ISL78211 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 18 gives this relationship.
V_DCR = I _o × DCR
(EQ. 18)
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 to 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 Equation 19:
R_n (T )
=
(R_series + R_ntc ) ⋅ R_par
R_series + R_ntc + R_par
(EQ. 19)
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. 20)
The inductor DCR is a function of the temperature and is
approximately given by Equation 21:
DCR(T ) = DCR_25C ⋅ (1 + 0.00393* (T − 25))
(EQ. 21)
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 Equation 22:
R_droop = G₁(T)⋅ DCR(T ) ⋅ k_droopamp
(EQ. 22)
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. 23)
where G_1target is the desired ratio of V_n/V_DCR. Therefore,
the temperature characteristics G₁ is described by
Equation 24:
G₁ (T)
=
(1+
G_{1t arg et}
0.00393* (T
−
25)
(EQ. 24)
For different G1 and NTC thermistor preference, Intersil
provides a design spreadsheet to generate the proper
value of R_ntc, R_series, R_par.
R_drp1 and R_drp2 (R₁₁ and R₁₂ in Figure 2) sets the droop
amplifier gain, according to Equation 25:
k_droopamp
=1+
R_drp2
R _drp1
(EQ. 25)
After determining R_s and R_n networks, use Equation 26
to calculate the droop resistances R_drp1 and R_drp2.
R_{drp 2}
=
(
R_droop
DCR ⋅ G1(25^o C)
− 1) ⋅ R_drp1
(EQ. 26)
20
FN7578.0
March 8, 2010