ISL62882 Datasheet, PDF (21/42 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62882, ISL62882B
Key Component Selection
R_BIAS
The ISL62882 uses a resistor (1% or better tolerance is
recommended) from the RBIAS pin to GND to establish
highly accurate reference current sources inside the IC.
Refer to Table 2 to select the resistance according to
desired configuration. Do not connect any other
components to this pin. Do not connect any capacitor to
the RBIAS pin as it will create instability.
Care should be taken in layout that the resistor is placed
very close to the RBIAS pin and that a good quality signal
ground is connected to the opposite side of the R_BIAS
resistor.
R_is and C_is
As Figures 1 thru 4 show, the ISL62882 needs the R_is
- C_is network across the ISUM+ and the ISUM- pins to
stabilize the droop amplifier. The preferred values are
R_is = 82.5Ω and C_is = 0.01µF. Slight deviations from the
recommended values are acceptable. Large deviations
may result in instability.
Inductor DCR Current-Sensing Network
Phase1 Phase2
Rsum
Rsum
ISUM+
L
L
DCR
DCR
Rntcs
Rp
Rntc
Ro
Ro
Cn ^Vcn
Ri ^ISUM-
Io
FIGURE 17. DCR CURRENT-SENSING NETWORK
Figure 17 shows the inductor DCR current-sensing
network for a 2-phase solution. An inductor current flows
through the DCR and creates a voltage drop. Each
inductor has two resistors in R_sum and R_o connected to
the pads to accurately sense the inductor current by
sensing the DCR voltage drop. The R_sum and R_o resistors
are connected in a summing network as shown, and feed
the total current information to the NTC network
(consisting of R_ntcs, R_ntc and R_p) and capacitor C_n. R_ntc
is a negative temperature coefficient (NTC) thermistor,
used to temperature-compensate the inductor DCR
change.
The inductor output side pads are electrically shorted in
the schematic, but have some parasitic impedance in
actual board layout, which is why one cannot simply
short them together for the current-sensing summing
network. It is recommended to use 1Ω~10Ω R_o to create
quality signals. Since R_o value is much smaller than the
rest of the current sensing circuit, the following analysis
will ignore it for simplicity.
The summed inductor current information is presented to
the capacitor C_n. Equations 14 thru 18 describe the
frequency-domain relationship between inductor total
current I_o(s) and C_n voltage V_Cn(s).
⎛
⎞
V_Cn(s)
=
⎜
⎜
⎜
⎝
-----------^R-----ⁿ---^t--^c---ⁿ----^e---^t-----------
R_n
t
cn
e
t
+
-^R----^s---^u---^m---
N
×
D_-----NC-----R_{---⎟⎟⎟}
⎠
× I_o(s) × A_cs(s)
(EQ. 14)
R_ntcnet
=
(_---R_{-----n---t--c---s----+-----}R_{----n----t--c---})_---×_-----R_----p--
R_ntcs + R_ntc + R_p
(EQ. 15)
A_cs(s)
=
----¹----+------^ω-----^s----L-----
1
+
------^s------
ω_sns
(EQ. 16)
ω_L
=
D_-----C_-----R_---
L
(EQ. 17)
ω_sns
=
---------------------------¹-----------------------------
-^R----ⁿ---^t--^c---ⁿ----^e---^t---^×------^R--------^s--^N----^u-------^m------
R_ntc
n
et
+
-^R----^s---^u---^m---
N
×
C_n
where N is the number of phases.
(EQ. 18)
Transfer function A_cs(s) always has unity gain at DC. The
inductor DCR value increases as the winding temperature
increases, giving higher reading of the inductor DC
current. The NTC R_ntc values decreases as its
temperature decreases. Proper selections of R_sum, R_ntcs,
R_p and R_ntc parameters ensure that V_Cn represent the
inductor total DC current over the temperature range of
interest.
There are many sets of parameters that can properly
temperature-compensate the DCR change. Since the
NTC network and the R_sum resistors form a voltage
divider, V_cn is always a fraction of the inductor DCR
voltage. It is recommended to have a higher ratio of V_cn
to the inductor DCR voltage, so the droop circuit has
higher signal level to work with.
A typical set of parameters that provide good
temperature compensation are: R_sum = 3.65kΩ, R_p =
11kΩ, R_ntcs = 2.61kΩ and R_ntc = 10kΩ (ERT-J1VR103J).
The NTC network parameters may need to be fine tuned
on actual boards. One can apply full load DC current
and record the output voltage reading immediately;
then record the output voltage reading again when the
board has reached the thermal steady state. A good
NTC network can limit the output voltage drift to within
2mV. It is recommended to follow the Intersil evaluation
21
FN6890.2
April 29, 2010