isl62391 Intersil Corporation, isl62391 Datasheet - Page 12

• Manufacturer Part Number: isl62391
• Description: High-efficiency, Triple-output System Power Supply Controller For Notebook Computers
• Manufacturer: Intersil Corporation

Theory of Operation
Three Output Controller
The ISL62391, ISL62392 generates three regulated output
voltages. Two are produced with switch-mode power supplies
(SMPS), and the third by a low dropout linear regulator (LDO).
An additional 5V LDO (PVCC) is used to power the chip
during operation, allowing the ISL62391, ISL62392 to regulate
all outputs from a single power source (VIN) with no need for
a separate quiescent supply. This makes the ISL62391,
ISL62392 an ideal choice as system regulator for notebook
PCs. Because the two SMPS channels are identical and
almost entirely independent, all conclusions drawn apply to
both channels unless otherwise noted.
Modulator and Switching Frequency
The ISL62391, ISL62392 modulator features Intersil’s R
technology, a hybrid of fixed frequency PWM and variable
frequency hysteretic control. Intersil’s R
simultaneously affect the PWM switching frequency and
PWM duty cycle in response to input voltage and output load
transients. The R
which is an analog representation of the output inductor
ripple current. The duty-cycle of V
and discharge current through a ripple capacitor, C
current through C
amplifier that measures the VIN and VO pin voltages. The
positive slope of V
The negative slope of V
Where g
V
V
RIPPLE CAPACITOR VOLTAGE C
RPOS
RNEG
FIGURE 23. MODULATOR WAVEFORMS DURING LOAD
=
=
m
g
g
is the gain of the transconductance amplifier.
m
m
⋅
⋅
TRANSIENT
(
V
V
OUT
IN
3
R
R
–
modulator synthesizes an AC signal, V
is provided by a transconductance
V
can be written as Equation 1:
OUT
R
)
can be written as Equation 2:
12
R
ERROR AMPLIFIER
VOLTAGE V
R
is the result of charge
WINDOW VOLTAGE V
3
(WRT V
technology can
COMP
PWM
COMP
)
R
ISL62391, ISL62392
. The
(EQ. 2)
(EQ. 1)
W
3
R
,
A window voltage V
amplifier output voltage V
which the ripple voltage V
V
GND pins. The V
window comparator in which V
voltage and V
shows PWM pulses being generated as V
V
is proportional to the slew rates of the positive and negative
slopes of V
between V
calculate the window size based on output voltage and
frequency set resistor.
V
The frequency can be expressed in Equation 4:
F
Inverting Equation 4 allows easy selection of R
desired F
R
For Equations 3 through 5:
Power-On Reset
The ISL62391, ISL62392 is disabled until the voltage at the
VIN pin has increased above the rising power-on reset
(POR) threshold. Conversely, the controller will be disabled
when the voltage at the VIN pin decreases below the falling
POR threshold.
In addition to VIN POR, the PVCC pin is also monitored. If its
voltage falls below 4.2V, the SMPS outputs will be shut
down. This ensures that there is sufficient BOOT voltage to
enhance the upper MOSFET.
EN, Soft-Start and PGOOD
The ISL62391, ISL62392 uses a digital soft-start circuit to
ramp the output voltage of each SMPS to the programmed
regulation setpoint at a predictable slew rate. The slew rate
of the soft-start sequence has been selected to limit the
in-rush current through the output capacitors as they charge
to the desired regulation voltage. When the EN pins are pulled
above their rising thresholds, the PGOOD Soft-Start Delay,
t
ramps to 0.6V in approximately 1.5ms and the PGOOD pin
goes to high impedance approximately 1.25ms after the FB
pin voltage reaches 0.6V.
SS
SW
W
W
W
W
, starts and the output voltage begins to rise. The FB pin
g
K = 1.7 x 10
D = V
is set by a resistor, R
and V
=
=
m
=
g
-------------------- -
K F
m
----------------- -
K R
= 1.66µs
⋅
⋅
⋅
SW
1
OUT
COMP
1
V
SW
W
R;
OUT
W
:
and V
W
it is inversely proportional to the voltage
/V
-10
IN
is the higher threshold voltage. Figure 23
⋅
thresholds. The PWM switching frequency
R,
(
1 D
COMP.
W
V
(±20%)
–
COMP,
is referenced with respect to the error
) R
⋅
W
COMP
R
Equation 3 illustrates how to
, connected across the FSET and
W
is compared. The amplitude of
and V
COMP
, creating an envelope into
W
signals feed into a
is the lower threshold
R
traverses the
December 22, 2008
W
for a
FN6666.4
(EQ. 3)
(EQ. 4)
(EQ. 5)