ISL62883_14 Datasheet, PDF (26/37 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs

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:
1_-5---.4_-2_---μ-4_---A-V_{-- – -}16_---.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.
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
NAME
LAYOUT CONSIDERATION
EP
GND 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, C6 R7,
8
FB
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 close
13
RTN
proximity of the controller for good decoupling.
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
26
FN6891.4
June 21, 2011