8
FN9107.3
June 30, 2005
FIGURE 2. TIMING DIAGRAM SHOWING VR_ON, VCC_CORE AND PGOOD FOR VCC_CORE, VCCP AND VCC_MCH
Operation
Initialization
Once the +5VDC supply voltage, as connected to the
ISL6217A VDD pin, reaches the Power-On Reset (POR)
rising threshold, the PWM drive signals are held in “high-
impedance state” or high impedance mode. This results in
both high and low side MOSFETs being held low. 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 signals are again driven to “high-impedance
state”.
The system signal, VR_ON is directly connected to the EN
pin of the ISL6217A. Once the voltage on the EN pin rises
above 2.0V, the chip is enabled and soft-start begins. The
EN pin of the ISL6217A is also used to reset the ISL6217A,
for cases when an undervoltage or overcurrent fault
condition has latched the IC off. A toggling of 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 ISL6217A regulates to the voltage on
the STV pin. This is referred to as the “Boot” voltage and is
labelled VBOOT in Figure 2. Once power good signals are
received from the Vccp and Vcc_mch regulators, the
ISL6217A will capture the VID code and regulate to this
command voltage within 3ms to 12ms. The PGOOD pin of
the ISL6217A is both an input and an output and is further
described in the “Fault Protection” section of this document.
Soft-Start Interval
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. Refer to Figure 2 and Figure 3.
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,
VDROOP. During Start-Up, when the voltage on SOFT is
less than the “Boot” voltage VBOOT, a small 30µA current
source, I1, is used to slowly ramp up the voltage on the soft-
start capacitor CSOFT. This slowly ramps up the reference
voltage for the controller, and therefore, controls the slew
rate of the output voltage. The STV pin is externally
programmable and sets the Start-Up, or “Boot” voltage,
VBOOT. The programming of this voltage level is explained
in the “STV, DSV and DRSV” section of this document.
The ISL6217A 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 ISL6227, Dual Voltage Regulator is an ideal choice
for the Vccp and Vcc_mch supplies.
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 ISL6217A is enabled
to capture the VID code and regulate to that command
voltage. Refer to Figure 2 and Figure 3. A second current
source, I2, is added to I1, after the initial Start-Up transition.
I2 is approximately 100µA, and raises the total SOFT pin
sinking and sourcing current to 130µA. This increased
current is used to increase the slew rate of the reference to
V
CC-CORE
VID
VR_ON / EN
PGOOD
Vccp / Vcc_mch
PGOOD
Vcc_core
>10us
t2
3ms to 12ms
V
BOOT
V
VID
< 3ms
-12%
Capture VID Code
t1
ISL6217A
