8
FN9101.6
August 6, 2007
Theory of Operation
Initialization
Once the +5VDC supply voltage (as connected to the
ISL6218 VDD pin) reaches the Power-On Reset (POR)
rising threshold, the PWM drive signals are held in
“Three-State” or high impedance mode. This results in both
the high side and low side MOSFETs being held off. Once
the supply voltage exceeds the POR rising threshold, the
controller will respond to a logic level high on the EN pin and
initiate the soft-start interval. If the supply voltage drops
below the POR falling threshold, POR shutdown is triggered
and the PWM outputs are again driven to “Three-State”.
The system signal, VR_ON is directly connected to the EN
pin of the ISL6218. Once the voltage on the EN pin rises
above 2.0V, the chip is enabled and soft-start begins. The
EN pin of the ISL6218 is also used to reset the ISL6218 for
cases when an undervoltage or overcurrent fault condition
has latched the IC off. Toggling the state of this pin to a level
below 1.0V will re-enable the IC. For the case of an
overvoltage fault, the VDD pin must be reset.
During start-up, the ISL6218 regulates to the voltage on the
STV pin. This is referred to as the “Boot” voltage and is
labeled VBOOT in Figure 2. Once power good signals are
received from the Vccp and Vcc_mch regulators, the
ISL6218 will capture the VID code and regulate, within 3ms
to 12ms, to this command voltage. The PGOOD pin of the
ISL6218 is both an input and an output and is further
described in “Fault Protection” on page 13.
Soft-Start Interval
Refer to Figure 2 and Figure 4. Once VDD rises above the
POR rising threshold and the EN pin voltage is above the
threshold of 2.0V, a soft-start interval is initiated. The voltage
on the EA+ pin is the reference voltage for the regulator. The
voltage on the EA+ pin is equal to the voltage on the SOFT
pin minus the “Droop” resistor voltage, V
DROOP
. During
start-up, when the voltage on SOFT is less than the “Boot”
voltage V
BOOT
, a 130µA current source I1, is used to slowly
ramp up the voltage on the soft-start capacitor C
SOFT
. This
slowly ramps up the reference voltage for the controller, and
controls the slew rate of the output voltage. The STV pin is
externally programmable and sets the start-up or “Boot”
voltage V
BOOT
. The programming of this voltage level is
explained in “STV, DSV and DRSV” on page 12.
The ISL6218 PGOOD pin is both an input and an output.
The system signal IMVP4_PWRGD is connected to power
good signals from the Vccp and Vcc_mch supplies. The
Intersil ISL6225 Dual Voltage Regulator is an ideal choice for
the Vccp and Vcc_mch supplies.
Refer to Figure 2 and Figure 4. Once the output voltage is
within the “Boot” level regulation limits and a logic high
PGOOD signal from the Vccp and Vccp_mch regulators is
received, the ISL6218 is enabled to capture the VID code
and regulate to that command voltage.
The “Droop” current source I
DROOP
, is proportional to load
current. This current source is used to reduce the reference
voltage on EA+ by the voltage drop across the “Droop”
resistor. A more in-depth explanation of “Droop” and the
sizing of this resistor can be found in “Droop Compensation”
on page 14.
FIGURE 2. TIMING DIAGRAM SHOWING VR_ON, VCC_CORE AND PGOOD FOR VCC_CORE, VCCP AND VCC_MCH
VID
VR_ON/EN
V
CC-CORE
PGOOD
VCCP/VCC-MCH
PGOOD
VCC-CORE
<3ms
-12%
t1
3ms TO 12ms
>10µs
t2
CAPTURE VID CODE
V
BOOT
V
VID
ISL6218
