ISL62882_14 Datasheet, PDF (27/42 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5™ Mobile CPUs and GPUs

ISL62882, ISL62882B
Optional Slew Rate Compensation Circuit For
1-Tick VID Transition
Rdroop
Rvid Cvid
FB
Ivid
Vcore
OPTIONAL
Idroop_vid
COMP
E/A
Σ
DAC
VDAC
X1
INTERNAL
TO IC
VIDs
VID<0:6>
RTN
VSSSENSE
VSS
VID<0:6>
Vfb
Ivid
Vcore
Idroop_vid
FIGURE 28. OPTIONAL SLEW RATE COMPENSATION CIRCUIT
FOR1-TICK VID TRANSITION
During a large VID transition, the DAC steps through the VIDs at a
controlled slew rate. For example, the DAC may change a tick
(12.5mV) per 2.5µs per, controlling output voltage V_core slew
rate at 5mV/µs.
Figure 28 shows the waveforms of 1-tick VID transition. During
1-tick VID transition, the DAC output changes at approximately
15mV/µs slew rate, but the DAC cannot step through multiple
VIDs to control the slew rate. Instead, the control loop response
speed determines V_core slew rate. Ideally, V_core will follow the FB
pin voltage slew rate. However, the controller senses the inductor
current increase during the up transition, as the I_{droop_vid}
waveform shows, and will droop the output voltage V_core
accordingly, making V_core slew rate slow. Similar behavior occurs
during the down transition.
To control V_core slew rate during 1-tick VID transition, one can
add the R_vid-C_vid branch, whose current I_vid cancels I_{droop_vid}.
When V_core increases, the time domain expression of the
induced I_droop change is
I_droop(t)
=
-^C---^o---^u----^t---^×-----^L---^L-
R_droop
×
-^d---^V----^c---^o---^r--^e-
dt
×
⎛
⎜
⎜
1
⎝
–
e
C^----_o^----_u^---–-_t^--t-×^-----L^---L^--_⎟⎞
⎟
⎠
(EQ. 38)
where C_out is the total output capacitance.
In the mean time, the R_vid-C_vid branch current I_vid time domain
expression is:
I_vid(t)
=
C_vid
×
d_----V_----f--b-
dt
×
⎛
⎜
⎜
1
⎝
–
e ^{R-----v---i--d---–-×---t--C----v---i--d--}_{⎟⎟⎞}
⎠
(EQ. 39)
It is desired to let I_vid(t) cancel I_{droop_vid}(t). So there are:
C_v
id
×
d_----V_----f--b-
dt
=
-^C---^o---^u----^t---^×-----^L---^L- × -^d---^V----^c---^o---^r--^e-
R_droop
dt
(EQ. 40)
and:
R_vid × C_vid = C_out × LL
(EQ. 41)
The result is expressed in Equation 42:
R_vid = R_droop
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. 42)
(EQ. 43)
For example: given LL = 1.9mΩ, R_droop = 2.87kΩ, C_out = 1710µF,
dV_core/dt = 5mV/µs and dV_fb/dt = 15mV/µs, Equation 42 gives
R_vid = 2.87kΩ and Equation 43 gives C_vid = 377pF.
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 effect on transient response.
Voltage Regulator Thermal Throttling
54µA 64µA
NTC
+
V_NTC
-
R_NTC
R_s
1.24V
SW1
VR_TT#
-
+
SW2
1.20V
INTERNAL TO
ISL62882
FIGURE 29. CIRCUITRY ASSOCIATED WITH THE THERMAL
THROTTLING FEATURE OF THE ISL62882
27
FN6890.4
June 21, 2011