FAN5070 Datasheet, PDF (8/14 Pages) Fairchild Semiconductor – High Performance Programmable Synchronous DC-DC Controller

FAN5070
Application Information
The FAN5070 Controller
The FAN5070 is a programmable synchronous DC-DC con-
troller IC. When designed around the appropriate external
components, the FAN5070 can be configured to deliver more
than 28A of output current, as appropriate for Intel’s
VRM8.5, and other processors. The FAN5070 functions as a
fixed frequency PWM step down regulator.
Main Control Loop
Refer to the FAN5070 Block Diagram on page 1. The
FAN5070 implements “summing mode control,” which is
different from both classical voltage-mode and current-mode
control. It provides superior performance to either by allowing
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 comparator which provides the input to the
digital control block. The signal conditioning section accepts
input from the DROOP (current feedback) and VFB (voltage
feedback) pins and sets up two controlling signal paths. The
first, the voltage control path, amplifies the difference
between the VFB signal and the reference voltage from the
DAC and presents the output to one of the summing amplifier
inputs. The second, current control path, takes the difference
between the DROOP and SW pins when the high-side
MOSFET is on, reproducing the voltage across the MOSFET
and thus the input current; it presents the resulting signal to
another input of the summing amplifier. These two signals are
then summed together. This output is then presented to a com-
parator looking at the oscillator ramp, which provides the
main PWM control signal to the digital control block.
The digital control block takes the analog comparator input
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.
There is an additional comparator in the analog control sec-
tion whose function is to set the point at which the FAN5070
current limit comparator disables the output drive signals to
the external power MOSFETs.
High Current Output Drivers
The FAN5070 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 power supply pin, V_CCP, is supplied from an
external 12V source through a series resistor. The resulting
voltage is sufficient to provide the gate to source drive to the
external MOSFETs required in order to achieve a low
R_DS,ON.
Internal Voltage Reference
The reference included in the FAN5070 is a precision band-
gap voltage reference. Its internal resistors are precisely
trimmed to provide a near zero temperature coefficient (TC).
Based on the reference is the output from an integrated 5-bit
DAC.
In the FAN5070, the DAC monitors the 5 voltage identifica-
tion pins, VID0-4, and scales the voltage from 1.050V to
1.825V in 25mV steps according to Table I.
Power Good (PWRGD)
The FAN5070 Power Good function is designed in accor-
dance with VRM8.5 and 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
be more than +14% from (V_VID + 80mV), or less than –14%
from (V_VID – 120mV). The Power Good flag provides no
other control function to the FAN5070.
Output Enable/Soft Start (ENABLE/SS)
The FAN5070 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.
Even if an enable is not required in the circuit, this pin
should have attached a capacitor (typically 100nF) to soft-
start the switching.
Over-Voltage Protection
The FAN5070 continuously monitors the output voltage for
protection against over-voltage conditions. If the voltage at
the VFB pin exceeds approximately 2.5V, an OVP circuit
forces the low-side MOSFET on, over-riding all other
conditions. The OVP circuit remains active, and the low-side
MOSFET remains on, until the VFB voltage drops below
approximately 2.1V. The OVP circuit is functional even
during startup; thus, protection is provided even during
startup with a shorted high-side MOSFET.
Oscillator
The FAN5070 oscillator section uses a fixed frequency of
operation of 300KHz.
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REV. 1.0.1 8/7/01