ISL62881_14 Datasheet, PDF (23/35 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

ISL62881, ISL62881B
where C_out is the total output capacitance.
In the meantime, the R_vid-C_vid branch current I_vid time domain
expression is as shown in Equation 32:
I_vid(t)
=
C_vid
×
d_----V_----f--b-
dt
×
⎛
⎜
⎜
1
⎝
–
e
R^-----_v^---_i^--_d^---–-×^--t---C^----_v^---_i^--_d^--_⎟⎞
⎟
⎠
(EQ. 32)
It is desired to let I_vid(t) cancel I_{droop_vid}(t). So there are:
C_vi
d
×
d_----V_----f--b-
dt
=
-^C---^o---^u----^t---^×-----^L---^L- × -^d---^V----^c---^o---^r--^e-
R_droop
dt
(EQ. 33)
and:
R_vid × C_vid = C_out × LL
(EQ. 34)
The result is:
R_vid = R_droop
(EQ. 35)
and:
C_vid
=
C_{----o---u----t---}×_-----L_---L_-
×
d_----V_{----c---o---r--e-}
-------^d----^t------
R_droop
d_----V_----f--b-
dt
(EQ. 36)
For example: given LL = 3mΩ, R_droop = 4.22kΩ, C_out = 1320µF,
dV_core/dt = 5mV/µs and dV_fb/dt = 15mV/µs, Equation 35 gives
R_vid = 4.22kΩ and Equation 36 gives C_vid = 227pF.
It’s recommended to select the calculated R_vid value and start
with the calculated C_vid value and tweak it on the actual board to
get the best performance.
During normal transient response, the FB pin voltage is held
constant, therefore is virtual ground in small signal sense. The
R_vid-C_vid network is between the virtual ground and the real
ground, and hence has no affect on transient response.
Voltage Regulator Thermal Throttling
Figure 24 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 the threshold voltage of 1.20V and the
comparator output is low. VR_TT# is pulled up by the external
resistor.
54µA 64µA
NTC
+
V_NTC
-
R_NTC
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:
-¹⁵---^.4-²---_μ-⁴---_A-^V-- – ¹-₆---^.0-²---_μ-⁰---_A-^V-- = 2.96k
(EQ. 37)
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 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---^.--²---⁰----^V--
60 μ A
–
15.6 k Ω
=
4.4 k Ω
(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.
23
FN6924.3
June 16, 2011