ISL62884CIRTZ Intersil, ISL62884CIRTZ Datasheet - Page 12
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ISL62884CIRTZ
Manufacturer Part Number
ISL62884CIRTZ
Description
IC REG PWM SGL PHASE 28TQFN
Manufacturer
Intersil
Datasheet
1.ISL62884CHRTZ.pdf
(30 pages)
Specifications of ISL62884CIRTZ
Applications
Controller, Intel IMVP-6
Voltage - Input
4.5 ~ 25 V
Number Of Outputs
1
Voltage - Output
0.0125 ~ 1.5 V
Operating Temperature
-40°C ~ 100°C
Mounting Type
Surface Mount
Package / Case
28-VQFN
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Part Number:
ISL62884CIRTZ
Manufacturer:
NA
Quantity:
20 000
Diode Emulation and Period Stretching
ISL62884C can operate in diode emulation (DE) mode to
improve light load efficiency. In DE mode, the low-side
MOSFET conducts when the current is flowing from
source to drain and doesn’t not allow reverse current,
emulating a diode. As shown in Figure 6, when LGATE is
on, the low-side MOSFET carries current, creating
negative voltage on the phase node due to the voltage
drop across the ON-resistance. The ISL62884C monitors
the current through monitoring the phase node voltage.
It turns off LGATE when the phase node voltage reaches
zero to prevent the inductor current from reversing the
direction and creating unnecessary power loss.
If the load current is light enough, as Figure 7 shows, the
inductor current will reach and stay at zero before the
next phase node pulse, and the regulator is in
discontinuous conduction mode (DCM). If the load
current is heavy enough, the inductor current will never
reach 0A, and the regulator is in CCM although the
controller is in DE mode.
VCRS
VCRS
VCRS
U G A T E
P H A S E
L G A T E
IL
IL
IL
I L
FIGURE 7. PERIOD STRETCHING
FIGURE 6. DIODE EMULATION
CCM/DCM BOUNDARY
VW
VW
VW
LIGHT DCM
DEEP DCM
12
ISL62884C
Figure 7 shows the operation principle in diode emulation
mode at light load. The load gets incrementally lighter in
the three cases from top to bottom. The PWM on-time is
determined by the VW window size, therefore is the
same, making the inductor current triangle the same in
the three cases. The ISL62884C clamps the ripple
capacitor voltage V
inductor current. It takes the COMP voltage longer to hit
V
inductor current triangles move further apart from each
other such that the inductor current average value is
equal to the load current. The reduced switching
frequency helps increase light load efficiency.
Start-Up Timing
With the controller's V
threshold, the start-up sequence begins when VR_ON
exceeds the 1.1V logic high threshold.
Figure 8 shows the typical start-up timing. The
ISL62884C uses digital soft-start to ramp up DAC to the
boot voltage of 1.2V at about 2.5mV/µs. Once the output
voltage is within 10% of the boot voltage for 13 PWM
cycles (43µs for frequency = 300kHz), CLK_EN# is
pulled low and DAC slews at 10mV/µs to the voltage set
by the VID pins. PGOOD is asserted high in
approximately 7ms. Similar results occur if VR_ON is tied
to V
V
Voltage Regulation and Load Line
Implementation
After the start sequence, the ISL62884C regulates the
output voltage to the value set by the VID inputs per
Table 1. The ISL62884C will control the no-load output
voltage to an accuracy of ±0.5% over the range of
0.75V to 1.5V. A differential amplifier allows voltage
sensing for precise voltage regulation at the
microprocessor die.
FIGURE 8. SOFT-START WAVEFORMS FOR CPU VR
CLK_EN#
crs
DD
PGOOD
VR_ON
, naturally stretching the switching period. The
DD
crosses the POR threshold.
VDD
DAC
, with the soft-start sequence starting 120µs after
APPLICATION
800µs
13 SWITCHING CYCLES
crs
2.5mV/µs
DD
in DE mode to make it mimic the
voltage above the POR
90%
10mV/µs
VBOOT
~7ms
VID
COMMAND
VOLTAGE
March 16, 2010
FN7591.0