ISL6313B_14 Datasheet, PDF (19/33 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers for Intel VR11 and AMD Applications

ISL6313B
Dynamic VID
Modern microprocessors need to make changes to their core
voltage as part of normal operation. They direct the ISL6313B
to do this by making changes to the VID inputs. The ISL6313B
is required to monitor the DAC inputs and respond to on-the-
fly VID changes in a controlled manner, supervising a safe
output voltage transition without discontinuity or disruption.
The DAC mode the ISL6313B is operating in determines
how the controller responds to a dynamic VID change.
INTEL DYNAMIC VID TRANSITIONS
When in Intel VR11 mode the ISL6313B checks the VID
inputs on the positive edge of an internal 5.5MHz clock. If a
new code is established and it remains stable for 3
consecutive readings (0.36µs to 0.54µs), the ISL6313B
recognizes the new code and changes the internal DAC
reference directly to the new level. The Intel processor
controls the VID transitions and is responsible for
incrementing or decrementing one VID step at a time. In VR11
mode, the ISL6313B will immediately change the internal
DAC reference to the new requested value as soon as the
request is validated, which means the fastest recommended
rate at which a bit change can occur is once every 1µs. If the
VID code is changed by more then one step at a time, the
DAC will try to track it at a 5.5MHz step rate. This will likely
cause an overcurrent or overvoltage fault.
AMD DYNAMIC VID TRANSITIONS
When running in AMD 5-bit or 6-bit modes of operation, the
ISL6313B responds differently to a dynamic VID change then
when in Intel VR11 mode. In the AMD modes the ISL6313B
still checks the VID inputs on the positive edge of an internal
5.5MHz clock. In these modes the VID code can be changed
by more than a 1-bit step at a time. If a new code is
established and it remains stable for 3 consecutive readings
(0.36µs to 0.54µs), the ISL6313B recognizes the change
and begins slewing the DAC in 6.25mV steps at a stepping
frequency of 345kHz until the VID and DAC are equal. Thus,
the total time required for a VID change, t_DVID, is dependent
only on the size of the VID change (ΔV_VID).
The time required for a ISL6313B-based converter in AMD 5-bit
DAC configuration to make a 1.1V to 1.5V reference voltage
change is about 186µs, as calculated using Equation 14.
t_DVID
=
------------¹-------------
345 × 10³
⋅
⎛
⎝
0^---Δ-.^-0^-V---0^--V-6^--I-2^-D---5^{--⎠⎞}
(EQ. 14)
VID “OFF” DAC CODES
Both the Intel VR11 and the AMD 5-bit VID tables include “Off”
DAC codes, which indicate to the controller to disable all
regulation. Recognition of these codes is slightly different in that
they must be stable for 4 consecutive readings of a 5.5MHz
clock (0.54µs to 0.72µs) to be recognized. Once an “Off” code
is recognized the ISL6313B latches off, and must be reset by
dropping the EN pin.
Compensating Dynamic VID Transitions
During a VID transition, the resulting change in voltage on
the FB pin and the COMP pin causes an AC current to flow
through the error amplifier compensation components from
the FB to the COMP pin. This current then flows through the
feedback resistor, R_FB, and can cause the output voltage to
overshoot or undershoot at the end of the VID transition. In
order to ensure the smooth transition of the output voltage
during a VID change, a VID-on-the-fly compensation
network is required. This network is composed of a resistor
and capacitor in series, R_DVC and C_DVC, between the DVC
and the FB pin.
VSEN
R_FB ^IDVC ^{= I}C
I_C
I_DVC
DVC
C_DVC
R_DVC
C_C
FB
R_C
COMP
x2
REF
C_REF
V_DAC + R_GND
-
+ _ERROR
AMPLIFIER
ISL6313B INTERNAL CIRCUIT
FIGURE 10. DYNAMIC VID COMPENSATION NETWORK
This VID-on-the-fly compensation network works by
sourcing AC current into the FB node to offset the effects of
the AC current flowing from the FB to the COMP pin during a
VID transition. To create this compensation current the
ISL6313B sets the voltage on the DVC pin to be 2x the
voltage on the REF pin. Since the error amplifier forces the
voltage on the FB pin and the REF pin to be equal, the
resulting voltage across the series RC between DVC and FB
is equal to the REF pin voltage. The RC compensation
components, R_DVC and C_DVC, can then be selected to
create the desired amount of compensation current.
The amount of compensation current required is dependant
on the modulator gain of the system, K1, and the error
amplifier R-C components, R_C and C_C, that are in series
between the FB and COMP pins. Use Equations 15, 16 and
17 to calculate the RC component values, R_DVC and C_DVC,
for the VID-on-the-fly compensation network. For these
equations: V_IN is the input voltage for the power train; V_PP is
the oscillator ramp amplitude (1.5V); and R_C and C_C are the
error amplifier R-C components between the FB and COMP
pins.
K1
=
-^V-----^I-^N----
V_PP
A
=
-----^K-----¹------
K1 – 1
(EQ. 15)
R_DVC = A × R_C
(EQ. 16)
C_DVC
=
C_-----C--
A
(EQ. 17)
19
FN6809.0
November 6, 2008