ISL6529A Datasheet, PDF (7/15 Pages) Intersil Corporation – Dual Regulator.Synchronous Rectified Buck PWM and Linear Power Controller

ISL6529A
NC (Pins 7, 8, 11, and 12), (Pins 6, 7, 8, 11, 13 and
15 MLFP)
No internal connection.
FB (Pin 9), (Pin 9 MLFP) and COMP (Pin 10), (Pin
10 MLFP).
FB and COMP are the available external pins of the error
amplifier. The FB pin is the inverting input of the error amplifier
and the COMP pin is the error amplifier output. These pins are
used to compensate the voltage-mode control feedback loop of
the standard synchronous rectified buck converter.
12VCC(Pin 13), (Pin 12 MLFP)
Provides bias voltage for the gate drivers.The voltage at this
pin is monitored for Power-On Reset (POR) purposes.
UGATE (Pin 14), (Pin 14 MLFP)
Connect UGATE to the upper MOSFET gate. This pin
provides the gate drive for the MOSFET.
Description
Operation Overview
The ISL6529A monitors and precisely controls two output
voltage levels. Refer to the Block Diagram, Simplified Power
System Diagram, and Typical Application Schematic on pp. 2–
3. The controller is intended for use in graphics cards or
embedded processor applications with 5V and 12V bias input
available. The IC integrates both a standard buck PWM
controller and a linear controller. The PWM controller is
designed to regulate the high current GPU voltage (V_OUT1).
The PWM controller regulates the output voltage to a level
programmed by a resistor divider. The linear controller is
designed to regulate the lower current local memory voltage
(V_OUT2) through an external N-Channel MOS pass transistor.
Initialization
The ISL6529A automatically initializes upon application of
input power. Special sequencing of the input supplies is not
necessary. The POR function continually monitors the input
bias supply voltage at the 5VCC and 12VCC pins. The POR
function initiates soft-start operation after these supply
voltages exceed their POR threshold voltages.
Soft-Start
The POR function initiates the digital soft-start sequence.
Both the linear regulator error amplifier and PWM error
amplifier reference inputs are forced to track a voltage level
proportional to the soft-start voltage. As the soft-start voltage
slews up, the PWM comparator regulates the output relative
to the tracked soft-start voltage, slowly charging the output
capacitor(s). Simultaneously, the linear output follows the
smooth ramp of the soft-start function into normal regulation.
Figure 1 shows the soft-start sequence of an ISL6529A
evaluation board powered by an ATX supply. Note the
uniform linear output voltage rise of the two ISL6529A output
voltages. Once the voltage on 5VCC crosses the POR
thresholds, both outputs begin their soft-start sequence. The
triangle waveform from the PWM oscillator is compared to
the rising error amplifier output voltage. As the error amplifier
voltage increases, the pulse-width on the PWM increases to
reach its steady-state duty cycle. The error amplifier
reference of the linear controller also rises relative to the
soft-start reference.
Figure 2 shows the controlled stepped output voltage rise
and associated charging current of a 390µF polymer
capacitor. By providing many small steps of current that
effectively charge the output capacitor, the potentially large
peak current resulting from a sudden, uncontrolled voltage
rise are eliminated.
The clock for the DAC producing the 30mV steps is
approximately 18.5kHz, so there is a 18.5kHz ripple current
component that lasts for the approximate 2.8ms start-up
interval. A few clock cycles are used for initialization to insure
that soft start begins near zero volts.
5VCC INPUT
3.3V INPUT
2.4V OUTPUT
1.5V OUTPUT
FIGURE 1. ATX SUPPLY POWERING AN ISL6529A
EVALUATION BOARD
7
FN9127.1
December 28, 2004