ISL6556B Datasheet, PDF (15/24 Pages) Intersil Corporation – Optimized Multi-Phase PWM Controller with 6-Bit DAC and Programmable Internal Temperature Compensation for VR10.X Application

ISL6556B
on-the-fly VID changes in a controlled manner. Supervising
the safe output voltage transition within the DAC range of the
processor without discontinuity or disruption is a necessary
function of the core-voltage regulator.
The ISL6556B checks the VID inputs six times every
switching cycle. If the VID code is found to have changed,
the controller waits half of a complete cycle before executing
a 12.5mV change. If during the half-cycle wait period, the
difference between DAC level and the new VID code
changes sign, no change is made. If the VID code is more
than 1 bit higher or lower than the DAC (not recommended),
the controller will execute 12.5mV changes six times per
cycle until VID and DAC are equal. It is for this reason that it
is important to carefully control the rate of VID stepping in 1-
bit increments.
In order to ensure the smooth transition of output voltage
during VID change, a VID step change smoothing network is
required for an ISL6556B based voltage regulator. This
network is composed by a 1kΩ internal resistor between the
output of DAC and the C_REF between REF pin to ground.
The selection of C_REF is based on the time duration for 1 bit
VID change and the allowable delay time.
Assuming the microprocessor controls the VID change at 1
bit every T_VID, the relationship between C_REF and T_VID is
given by Equation 9.
C_REF = 0.004X T_VID
(EQ. 9)
As an example, for a VID step change rate of 5µs per bit, the
value of C_REF is 22nF based on Equation 9.
Temperature Compensation
The MOSFET r_DS(ON) varies in proportion to varying
temperature. This means that a circuit using r_DS(ON) to
sense channel current is subject to a corresponding error in
current measurement. In order to compensate for this
temperature-related error, a temperature compensation
circuit is provided within ISL6561. This circuit senses the
internal IC temperature and, based on a resistor-selectable
scaling factor, adjust the droop current flow to the FB pin.
When the TCOMP resistor is properly selected, the droop
current can accurately represent the load current to achieve
a linear, temperature-independant load line.
The value of the Tcomp resistor can be determined using
Equation 10.
R_TCOMP
=
---------^α------------
K_T K_TC
(EQ. 10)
In Equation 10, K_T is the temperature coupling coefficient
between the ISL6561 and the lower MOSFET. It represents
how closely the controller temperature tracks the lower
MOSFET temperature. The value of K_T is typically between
75% and 100%. K_TC is the temperature dependant
transconductance of internal compensation circuit. Its value
15
is designed as 1µA/V/°C. The temperature coefficient of
MOSFET r_DS(ON) is given by α . This is the ratio of the
change in resistance and the change in temperature.
Resistance is normalized to the value at 25°C and the value
of α is typically between 0.35%/°C and 0.50%/°C.
According to Equation 10, a voltage regulator with 80%
thermal coupling coefficient between the controller and lower
MOSFET and 0.4%/°C temperature coefficient of MOSFET
r_DS(ON) requires a 5kΩ TCOMP resistor.
Initialization
Prior to initialization, proper conditions must exist on the
enable inputs and VCC. When the conditions are met, the
controller begins soft-start. Once the output voltage is within
the proper window of operation, PGOOD asserts logic.
Enable and Disable
While in shutdown mode, the PWM outputs are held in a
high-impedance state to assure the drivers remain off. The
following input conditions must be met before the ISL6556B
is released from shutdown mode.
1. The bias voltage applied at VCC must reach the internal
power-on reset (POR) rising threshold. Once this
threshold is reached, proper operation of all aspects of
the ISL6556B is guaranteed. Hysteresis between the
rising and falling thresholds assure that once enabled,
the ISL6556B will not inadvertently turn off unless the
bias voltage drops substantially (see Electrical
Specifications).
ISL6556B INTERNAL CIRCUIT EXTERNAL CIRCUIT
VCC
+12V
POR
CIRCUIT
SOFT-START
AND
FAULT LOGIC
ENABLE
COMPARATOR
+
-
10.7kΩ
EN
1.40kΩ
1.24V
ENLL
(ISL6556BCR ONLY)
FIGURE 8. POWER SEQUENCING USING THRESHOLD-
SENSITIVE ENABLE (EN) FUNCTION
2. The ISL6556B features an enable input (EN) for power
sequencing between the controller bias voltage and
another voltage rail. The enable comparator holds the
ISL6556B in shutdown until the voltage at EN rises above
1.24V. The enable comparator has about 100mV of
hysteresis to prevent bounce. It is important that the
FN9097.4
December 28, 2004