ISL62883 Datasheet, PDF (27/39 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62883, ISL62883B
Figure 26 shows the thermal throttling feature with
hysteresis. An NTC network is connected between the
NTC pin and GND. At low temperature, SW1 is on and
SW2 connects to the 1.20V side. The total current
flowing out of the NTC pin is 60µA. The voltage on NTC
pin is higher than threshold voltage of 1.20V and the
comparator output is low. VR_TT# is pulled up by the
external resistor.
When temperature increases, the NTC thermistor
resistance decreases so the NTC pin voltage drops. When
the NTC pin voltage drops below 1.20V, the comparator
changes polarity and turns SW1 off and throws SW2 to
1.24V. This pulls VR_TT# low and sends the signal to
start thermal throttle. There is a 6µA current reduction
on NTC pin and 40mV voltage increase on threshold
voltage of the comparator in this state. The VR_TT#
signal will be used to change the CPU operation and
decrease the power consumption. When the temperature
drops down, the NTC thermistor voltage will go up. If
NTC voltage increases to above 1.24V, the comparator
will flip back. The external resistance difference in these
two conditions is expressed in Equation 49:
-¹⁵---^.4--²--_μ-⁴---_A-^V-- – ¹-₆---^.0--²--_μ-⁰---_A-^V-- = 2.96k
(EQ. 49)
One needs to properly select the NTC thermistor value
such that the required temperature hysteresis correlates
to 2.96kΩ resistance change. A regular resistor may
need to be in series with the NTC thermistor to meet the
threshold voltage values.
For example, given Panasonic NTC thermistor with
B = 4700, the resistance will drop to 0.03322 of its
nominal at +105°C, and drop to 0.03956 of its nominal
at +100°C. If the required temperature hysteresis is
+105°C to +100°C, the required resistance of NTC will
be:
------------------²---^.--⁹---⁶----^k---^Ω--------------------
(0.03956 – 0.03322)
=
467 k Ω
(EQ. 50)
Therefore a larger value thermistor, such as 470k NTC
should be used.
At +105°C, 470kΩ NTC resistance becomes
(0.03322×470kΩ) = 15.6kΩ. With 60µA on the NTC pin,
the voltage is only (15.6kΩ×60µA) = 0.937V. This value
is much lower than the threshold voltage of 1.20V.
Therefore, a regular resistor needs to be in series with
the NTC. The required resistance can be calculated by
Equation 51:
1_----._--2_---0_----V_--
60 μ A
–
15.6 k Ω
=
4.4 k Ω
(EQ. 51)
4.42k is a standard resistor value. Therefore, the NTC
branch should have a 470k NTC and 4.42k resistor in
series. The part number for the NTC thermistor is
ERTJ0EV474J. It is a 0402 package. The NTC thermistor
will be placed in the hot spot of the board.
Current Balancing
Refer to Figures 1 and 2. The ISL62883 achieves current
balancing through matching the ISEN pin voltages. R_s
and C_s form filters to remove the switching ripple of the
phase node voltages. It is recommended to use rather
long R_sC_s time constant such that the ISEN voltages
have minimal ripple and represent the DC current flowing
through the inductors. Recommended values are
R_s = 10kΩ and C_s = 0.22µF.
Layout Guidelines
Table 5 shows the layout considerations. The designators
refer to the reference design shown in Figure 27.
TABLE 5. LAYOUT CONSIDERATION
PIN
EP
NAME
GND
LAYOUT CONSIDERATION
Create analog ground plane underneath
the controller and the analog signal
processing components. Don’t let the
power ground plane overlap with the
analog ground plane. Avoid noisy
planes/traces (e.g.: phase node) from
crossing over/overlapping with the
analog plane.
1 PGOOD No special consideration
2
PSI# No special consideration
3
RBIAS Place the Rbias resistor (R16) in general
proximity of the controller. Low
impedance connection to the analog
ground plane.
4 VR_TT# No special consideration
5
NTC The NTC thermistor (R9) needs to be
placed close to the thermal source that is
monitor to determine thermal throttling.
Usually it’s placed close to Phase-1 high-
side MOSFET.
6
VW Place the capacitor (C4) across VW and
COMP in close proximity of the controller
7
COMP Place the compensator components (C3,
8
FB
C6 R7, R11, R10 and C11) in general
proximity of the controller.
9 ISEN3/FB2 A capacitor (C7) decouples it to VSUM-.
Place it in general proximity of the
controller.
An optional capacitor is placed between
this pin and COMP. (It’s only used when
the controller is configured 2-phase).
Place it in general proximity of the
controller.
10 ISEN2 A capacitor (C9) decouples it to VSUM-.
Place it in general proximity of the
controller.
11 ISEN1 A capacitor (C10) decouples it to VSUM-
. Place it in general proximity of the
controller.
12 VSEN Place the VSEN/RTN filter (C12, C13) in
13
RTN
close proximity of the controller for good
decoupling.
27
FN6891.2
February 25, 2010