ISL62881C Datasheet, PDF (19/37 Pages) Intersil Corporation – Single-Phase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62881C, ISL62881D
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_ntcnet + R_sum
×
D
C
⎞
R⎟
⎠
× I_o(s) × A_cs(s)
(EQ. 7)
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. 8)
(EQ. 9)
ω_L
=
D_-----C_-----R_---
L
(EQ. 10)
ω_sns
=
---------------------------¹----------------------------
-^R----ⁿ---^t--^c---ⁿ----^e---^t---^×-----^R----^s---^u----^m--
R_ntcnet + R_sum
×
C_n
(EQ. 11)
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 represents the
inductor total DC current over the temperature range of
interest.
There are many sets of parameters that can properly
temperature-compensate the DCR change. Since the
NTC network and the R_sum resistors form a voltage
divider, V_cn is always a fraction of the inductor DCR
voltage. It is recommended to have a higher ratio of V_cn
to the inductor DCR voltage, so the droop circuit has
higher signal level to work with.
A typical set of parameters that provide good
temperature compensation are: R_sum = 1.82kΩ,
R_p = 11kΩ, R_ntcs = 2.61kΩ and R_ntc = 10kΩ
(ERT-J1VR103J). The NTC network parameters may need
to be fine tuned on actual boards. One can apply full load
DC current and record the output voltage reading
immediately; then record the output voltage reading
again when the board has reached the thermal steady
state. A good NTC network can limit the output voltage
drift to within 2mV. It is recommended to follow the
Intersil evaluation board layout and current-sensing
network parameters to minimize engineering time.
V_Cn(s) also needs to represent real-time I_o(s) for the
controller to achieve good transient response. Transfer
function A_cs(s) has a pole ω_sns and a zero ω_L. One needs
to match ω_L and ω_sns so A_cs(s) is unity gain at all
frequencies. By forcing ω_L equal to ω_sns and solving for
the solution, Equation 12 gives C_n value.
C_n = --_R--------_n-------_t---c------_n--------_e------_t------_×----------_R----^L----_s------_u--------_m--------_×-----_D----_C-----_R---
R_ntcnet + R_sum
i_o
(EQ. 12)
V_o
FIGURE 14. DESIRED LOAD TRANSIENT RESPONSE
WAVEFORMS
i_o
V_o
FIGURE 15. LOAD TRANSIENT RESPONSE WHEN C_n IS
TOO SMALL
i_o
V_o
FIGURE 16. LOAD TRANSIENT RESPONSE WHEN C_n IS
TOO LARGE
For example, given R_sum = 1.82kΩ, R_p = 11kΩ,
R_ntcs = 2.61kΩ, R_ntc = 10kΩ, DCR = 1.3mΩ and
L = 0.56µH, Equation 12 gives C_n = 0.31µF.
Assuming the compensator design is correct, Figure 14
shows the expected load transient response waveforms if
C_n is correctly selected. When the load current I_core has
a square change, the output voltage V_core also has a
square response.
If C_n value is too large or too small, V_Cn(s) will not
accurately represent real-time I_o(s) and will worsen the
transient response. Figure 15 shows the load transient
response when C_n is too small. V_core will sag excessively
upon load insertion and may create a system failure.
Figure 16 shows the transient response when C_n is too
large. V_core is sluggish in drooping to its final value.
There will be excessive overshoot if load insertion occurs
during this time, which may potentially hurt the CPU
reliability.
19
FN7596.0
March 8, 2010