ISL62881 Datasheet, PDF (23/35 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62881, ISL62881B
54µ 64µ
NTC
+
V_{NTC RNTC}
-
R_s
1.24V
SW1
VR_TT#
-
+
SW2
1.20V
INTERNAL TO
ISL62881
FIGURE 24. CIRCUITRY ASSOCIATED WITH THE
THERMAL THROTTLING FEATURE OF THE
ISL62881
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 shown in Equation 37:
1_----._--2_---4_----V_--
54 μ A
–
1_----._--2_---0_----V_--
60 μ A
=
2.96 k
(EQ. 37)
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 as shown in Equation 38:
------------------²---^.--⁹---⁶----^k---^Ω--------------------
(0.03956 – 0.03322)
=
467 k Ω
(EQ. 38)
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 39:
1_-6---.0_--2_--μ-0_---A-V_{-- – 15.6kΩ = 4.4kΩ}
(EQ. 39)
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. NTC thermistor will
be placed in the hot spot of the board.
Layout Guidelines
Table 5 shows the layout considerations. The
designators refer to the reference designs shown in
Figures 25 and 26.
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.
NAME
GND
TABLE 5. LAYOUT CONSIDERATIONS
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.
CLK_EN# No special consideration.
PGOOD No special consideration
RBIAS
VR_TT#
NTC
VW
COMP
FB
Place the R_BIAS resistor (R₁₆) in general proximity of the controller. Low impedance connection to the analog ground
plane.
No special consideration.
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.
Place capacitor (C4) across VW and COMP in close proximity of the controller.
Place compensator components (C3, C5, C6 R7, R11, R10 and C11) in general proximity of the controller.
23
FN6924.0
October 26, 2009