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参数资料
| 型号： | FAN5061M |
| 厂商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 345 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封装： | SOIC-20 |
| 文件页数： | 5/18页 |
| 文件大小： | 149K |
| 代理商： | FAN5061M |
FAN5061
13
Pr
eliminar
y
Specication
is pulled LOW, the DAC scales the reference from 1.30V to
2.05V in 50mV increments. All VID codes are available, includ-
ing those below 1.80V.
Power Good (PWRGD)
The FAN5061 Power Good function is designed in accor-
dance with the Pentium II DC-DC converter specications
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 ag provides no other
control function to the FAN5061.
Output Enable/Soft Start (ENABLE/SS)
The FAN5061 will accept an open collector/TTL signal for
controlling the output voltage. The low state disables the output
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 softstart the
switching.
Over-Voltage Protection
The FAN5061 constantly monitors the output voltage for pro-
tection against over-voltage conditions. If the voltage at the
VFB pin exceeds the selected program voltage, an over-volt-
age condition is assumed and the FAN5061 disables the out-
put drive signal to the external high-side MOSFET. The DC-
DC converter returns to normal operation after the output
voltage returns to normal levels.
Oscillator
The FAN5061 oscillator section uses a xed frequency of
operation of 300KHz.
Design Considerations and Component
Selection
Additional information on design and component selection
may be found in Fairchild’s Application Note 57.
MOSFET Selection
This application requires N-channel Logic Level Enhancement
Mode Field Effect Transistors. Desired characteristics are as
follows:
Low Static Drain-Source On-Resistance, RDS,ON < 20m
(lower is better)
Low gate drive voltage, VGS = 4.5V rated
Power package with low Thermal Resistance
Drain-Source voltage rating > 15V.
The on-resistance (RDS,ON) is the primary parameter for
MOSFET selection. The on-resistance determines the power
dissipation within the MOSFET and therefore signicantly
affects the efciency of the DC-DC Converter. For details
and a spreadsheet on MOSFET selection, refer to Applica-
tions Bulletin AB-8.
Inductor Selection
Choosing the value of the inductor is a tradeoff between
allowable ripple voltage and required transient response. The
system designer can choose any value within the allowed
minimum to maximum range in order to either minimize ripple
or maximize transient performance. The rst order equation
(close approximation) for minimum inductance is:
where:
Vin = Input Power Supply
Vout = Output Voltage
f = DC/DC converter switching frequency
ESR = Equivalent series resistance of all output capacitors in
parallel
Vripple = Maximum peak to peak output ripple voltage budget.
The rst order equation for maximum allowed inductance is:
where:
Co = The total output capacitance
Ipp = Maximum to minimum load transient current
Vtb = The output voltage tolerance budget allocated to load
transient
Dm = Maximum duty cycle for the DC/DC converter (usually
95%).
Some margin should be maintained away from both Lmin and
Lmax. Adding margin by increasing L almost always adds
expense since all the variables are predetermined by system
performance except for CO, which must be increased to
increase L. Adding margin by decreasing L can be done by
purchasing capacitors with lower ESR. The FAN5061 pro-
vides signicant cost savings for the newer CPU systems
that typically run at high supply current.
FAN5061 Short Circuit Current Characteristics
The FAN5061 protects against output short circuit on the
core supply by turning off both the high-side and low-side
MOSFETs. The FAN5061 short circuit current characteristic
includes a hysteresis function that prevents the DC-DC
converter from oscillating in the event of a short circuit. The
short circuit limit is set with the RS resistor, as given by the
formula
Lmin
(Vin – Vout)
f
x
Vout
Vin
x
ESR
Vripple
=
Lmax
(Vin – Vout) Dm Vtb
Ipp2
= 2CO
RS
ISC *RDS, on
IDetect
=
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