ISL6539 Intersil Corporation, ISL6539 Datasheet - Page 9

• Manufacturer Part Number: ISL6539
• Description: Wide Input Range Dual PWM Controller with DDR Option
• Manufacturer: Intersil Corporation

Theory of Operation
Operation
The ISL6539 is a dual channel PWM controller intended for
use in power supplies for graphic chipsets, SDRAM, DDR
DRAM, or other power applications. The IC integrates two
control circuits for two synchronous buck converters. The
output voltage of each controller can be set in the range of
0.9V to 5.5V by an external resistive divider.
The synchronous buck converters can operate from either
an unregulated DC source with a voltage ranging from 5.0V
to 15V, or from a regulated system rail of 3.3V or 5V. In either
operational mode the controller is biased from the +5V
source.
The controllers operate in the current mode with input
voltage feed-forward which simplifies feedback loop
compensation and rejects input voltage variation. An
integrated feedback loop compensation dramatically
reduces the number of external components.
The ISL6539 has a special means to rearrange its internal
architecture into a complete DDR solution. When the DDR
pin is set high, the second channel can provide the capability
to track the output voltage of the first channel. The buffered
reference voltage required by DDR memory chips is also
provided.
Initialization
The ISL6539 initializes if at least one of the enable pins is
set high. The Power-On Reset (POR) function continually
monitors the bias supply voltage on the VCC pin, and
initiates soft-start operation when EN1 or EN2 is high after
the input supply voltage exceeds 4.45V. Should this voltage
drop lower than 4.14V, the POR disables the chip.
Soft-Start
When soft-start is initiated, the voltage on the SOFT pin of
the enabled channel starts to ramp up gradually with the
internal 5
output voltage follows the soft-start voltage.
When the SOFT pin voltage reaches 0.9V, the output voltage
comes into regulation. When the SOFT voltage reaches
1.5V, the power good (PGOOD) is enabled. The soft-start
process is depicted in Figure 3.
µ
A current charging the soft-start capacitor. The
9
ISL6539
Even though the soft-start pin voltage continues to rise after
reaching 1.5V, this voltage does not affect the output
voltage.
The soft-start time (the time from the moment when EN
becomes high to the moment when PGOOD is reported) is
determined by the following equation:
The time it takes the output voltage to come into regulation
can be obtained from the following equation.
During soft-start, before the PGOOD pin is enabled, the
undervoltage protection is prohibited. The overvoltage and
overcurrent protection functions are enabled.
If the output capacitor has residue voltage before start up,
both lower and upper MOSFETs are in off-state until the soft-
start capacitor charges equal the VSEN pin voltage. This will
ensure the output voltage starts from its existing voltage
level.
Output Voltage Program
The output voltage of either channel is set by a resistive
divider from the output to ground. The center point of the
divider is connected to the VSEN pin as shown in Figure 4.
The output voltage value is determined by the following
equation.
where 0.9V is the value of the internal reference. The VSEN
pin voltage is also used by the controller for the power good
function and to detect Undervoltage and Overvoltage
conditions.
T
T
V
S OFT
R ISE
O
=
3
1
2
4
0.9 V
--- --- --- --- --- --- --- --- --- --- --- --- --- --- --- -
=
=
Ch1 5.0V
Ch3 1.0V
0.6 T
1.5 V C sof t
--- --- --- --- -- --- --- --- --- --- --- --
•
×
(
R 2
R1
5µ A
0.9V
PGOOD
×
SOFT
+
EN
VOUT
1.5V
R 2
FIGURE 3. START UP
)
Ch2 2.0V
Ch4 5.0V
SOF T
M1.00ms