FAN5066 Datasheet, PDF (11/18 Pages) Fairchild Semiconductor – Ultra Low Voltage Synchronous DC-DC Controller

PRODUCT SPECIFICATION
FAN5066
Application Information
The FAN5066 Controller
The FAN5066 is a programmable synchronous DC-DC con-
troller IC. When designed around the appropriate external
components, the FAN5066 can be configured to deliver more
than 19A of output current. The FAN5066 functions as a
fixed frequency PWM step down regulator.
Main Control Loop
Refer to the FAN5066 Block Diagram on page 1. The
FAN5066 implements “summing mode control”, which is
different from both classical voltage-mode and current-mode
control. It provides superior performance to either by allow-
ing a large converter bandwidth over a wide range of output
loads.
The control loop of the regulator contains two main sections:
the analog control block and the digital control block. The
analog section consists of signal conditioning amplifiers
feeding into a set of comparators which provide the inputs to
the digital control block. The signal conditioning section
accepts inputs from the IFB (current feedback) and VFB
(voltage feedback) pins and sets up two controlling signal
paths. The first, the voltage control path, amplifies the differ-
ence between the VFB signal the reference voltage from the
DAC and presents the output to one of the summing ampli-
fier inputs. The second, current control path, takes the differ-
ence between the IFB and VFB pins and presents the
resulting signal to another input of the summing amplifier.
These two signals are then summed together with the slope
compensation input from the oscillator. This output is then
presented to a comparator, which provides the main PWM
control signal to the digital control block.
The digital control block takes the analog comparator inputs
and the main clock signal from the oscillator to provide the
appropriate pulses to the HIDRV and LODRV output pins.
These two outputs control the external power MOSFETs.
The digital block utilizes high speed Schottky transistor
logic, allowing the FAN5066 to operate at clock speeds as
high as 1MHz.
There are additional comparators in the analog control sec-
tion whose function is to set the point at which the FAN5066
enters its pulse skipping mode during light loads, as well as
the point at which the current limit comparator disables the
output drive signals to the external power MOSFETs.
High Current Output Drivers
The FAN5066 contains two identical high current output
drivers that utilize high speed bipolar transistors in a push-
pull configuration. The drivers’ power and ground are sepa-
rated from the chip’s power and ground for switching noise
immunity. The HIDRV driver has a power supply pin,
VCCQP, which is supplied from an external 12V source
through a series resistor or from a charge-pump circuit
powered from 5V if 12V is not available. The LODRV driver
has a power supply pin, VCCP, which can be supplied from
either the 12V or 5V source. The resulting voltages are suffi-
cient to provide the gate to source drive to the external
MOSFETs required in order to achieve a low RDS,ON.
Power Good (PWRGD)
The FAN5066 Power Good function is designed in accor-
dance with the Pentium II DC-DC converter specifications
and provides a continuous voltage monitor on the VFB pin.
The circuit compares the VFB signal to the VREF voltage
and outputs an active-low interrupt signal to the CPU should
the power supply voltage deviate more than ±12% of its
nominal setpoint. The Power Good flag provides no other
control function to the FAN5066.
Output Enable (ENABLE)
The FAN5066 will accept an open collector/TTL signal for
controlling the output voltage. The low state disables the out-
put voltage. When disabled, the PWRGD output is in the low
state. If an enable is not required in the circuit, this pin may
be left open.
Over-Voltage Protection
The FAN5066 constantly monitors the output voltage for
protection against over voltage conditions. If the voltage at
the VFB pin exceeds 20% of the selected program voltage,
an over-voltage condition is assumed and the FAN5066 dis-
ables the output drive signal to the external MOSFETs. The
DC-DC converter returns to normal operation after the fault
has been removed.
Over-Current Protection
Current sense is implemented in the FAN5066 to reduce the
duty cycle of the output drive signal to the MOSFETs when
an over-current condition is detected. The voltage drop
created by the output current flowing across a sense resistor
is presented to an internal comparator. When the voltage
developed across the sense resistor exceeds the 120mV com-
parator threshold voltage, the FAN5066 reduces the output
duty cycle to help protect the power devices. The DC-DC
converter returns to normal operation after the fault has been
removed.
Oscillator
The FAN5066 oscillator section uses a fixed current capaci-
tor charging configuration. An external capacitor (CEXT) is
used to set the oscillator frequency between 80KHz and
1MHz. This scheme allows maximum flexibility in choosing
external components.
REV. 2.1.4 11/13/01
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