ISL62883C Datasheet, PDF (23/43 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62883C
output voltage overshoots. The ISL62883C will turn off
LGATE1 and LGATE2 when COMP is being clamped. All
the low-side MOSFETs in the power stage will be turned
off. When the output voltage has reached its peak and
starts to come down, the COMP voltage starts to rise and
is no longer clamped. The ISL62883C will resume normal
PWM operation.
When PSI# is low, indicating a low power state of the
CPU, the controller will disable the overshoot reduction
function as large magnitude transient event is not
expected and overshoot is not a concern.
While the overshoot reduction function reduces the
output voltage overshoot, energy is dissipated on the
low-side MOSFET, causing additional power loss. The
more frequent transient event, the more power loss
dissipated on the low-side MOSFET. The MOSFET may
face severe thermal stress when transient events
happen at a high repetitive rate. User discretion is
advised when this function is enabled.
Key Component Selection
R_BIAS
The ISL62883C 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.
Inductor DCR Current-Sensing Network
Phase1 Phase2 Phase3
Rsum
Rsum
Rsum
ISUM+
L
L
L
DCR
DCR
DCR
Rntcs
Rp
Rntc
Ro
Ro
Ro
Cn Vcn
Ri ^ISUM-
Io
FIGURE 17. DCR CURRENT-SENSING NETWORK
Figure 17 shows shows the inductor DCR current-
sensing network for a 3-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 1W~10W 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 19 thru 23 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_nt
cn
e
t
+
-^R----^s---^u---^m---
N
×
D_-----CN_-----R_{---⎟⎟⎟}
⎠
× I_o(s) × A_cs(s)
(EQ. 19)
R_ntcnet
=
(_---R_{-----n---t--c---s----+-----}R_{----n----t--c---})_---×_-----R_----p--
R_ntcs + R_ntc + R_p
A_cs(s)
=
----¹----+-----^{ω------s----L-----}
1 + _ω-----_s-^s--_n---_s-
(EQ. 20)
(EQ. 21)
ω_L
=
D_-----C_-----R_---
L
(EQ. 22)
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. 23)
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.
23
FN7557.1
March 18, 2010