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PDF FAN7686 Data sheet ( Hoja de datos )
| Número de pieza | FAN7686 | |
| Descripción | PC Power Supply Output Monitoring IC | |
| Fabricantes | Fairchild Semiconductor | |
| Logotipo |  |
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FAN7685/FAN7686/FAN7687
PC Power Supply Output Monitoring IC
Features
• PC Power Supply Outputs Supervisory Circuitry
• Few External Components
• Over Voltage Protection for 3.3V, 5V and 12V Outputs
• Under Voltage Protection for 3.3V, 5V and 12V Outputs
• Over Current Protection for 3.3V, 5V and 12V Outputs
• Dual Over Current Portection for 12V Outputs(FAN7687)
• Fault Protection Output With Open Drain Output
• Open Drain Power Good Output
• 300ms Power Good Delay
• 38ms PSON On/Off Delay
• 73us Debounce
• 2.3ms PSON to FPO Turn Off Delay
• Latch Function Controlled by PSON
Typical Application
• PC Switching Mode Power Supply
Description
The FAN7685/FAN7686/FAN7687 is a complete output
supervisory circuitry intended for use in the secondary side
of the switched mode power supply. It provides overvoltage
protection (OVP), undervoltage protection(UVP), overcur-
rent protection (OCP), and power good signal generator to
monitor and control the outputs of the switching power sup-
ply system. Remote on/off(PSON) control and some preci-
sion protection features are also implemented.
It directly senses all the output rails for OVP, UVP, and OCP
without external divider resistors. As for output control,
power good output(PGO) and fault protection output(FPO)
are included. The FAN7685/FAN7686/FAN7687 offers a
simple and cost effective solution with minimum number of
external components and greatly reduces PCB board space
for power supply.
14DIP
1
14SOP
1
©2004 Fairchild Semiconductor Corporation
Rev. 1.1.0
1 page  
FAN7685/FAN7686/FAN7687
Electrical Characteristics(VCC = 5V, Ta=25°C, unless otherwise specified)
Over Voltage Protection, Under Voltage Protection and FPO
Parameter
Test Condition
VS33
FAN7687A
Over Voltage Threshold
VS5
FAN7687A
VS12,
VCC12
FAN7687A(VS12)
FAN7687A(VCC12)
VS33
FAN7687A
Under Voltage Threshold
VS5
FAN7687A
Ratio of Current Sense Sink Current to
Current Sense Setting Pin(RI) Source
Current
Offset Voltage of OCP Comparator
Leakage Current(FPO)
Low Level Output Voltage(FPO)
VS12,
VCC12
FAN7687A(VS12)
FAN7687A(VCC12)
Iref
Resistor at RI=30kΩ,
0.1% Resistor
Voffset
ILKGI
VOLI
VPSON=0V
VFPO = 5V
IFPO=10mA
IFPO=30mA
Min.
3.9
3.77
5.8
5.71
13.3
13.71
13.42
2.55
2.88
4.1
4.37
8.8
10.5
10.21
7.6
-5
-
-
-
Typ.
4.1
6.1
13.8
2.69
4.3
9.3
8
-
-
-
-
Max.
4.3
4.06
6.4
6.16
14.3
14.79
14.5
2.83
3.02
4.5
4.58
9.8
11.0
10.71
8.4
5
5
0.3
0.7
Unit
V
V
mV
uA
V
PGI and PGO
Input Threshold Voltage(PGI)
Leakage Current(PGO)
Low Level Output Voltage(PGO)
VPGI
ILKG2
VOL2
VPGO = 5V
IPGO=10mA
1.16
-
-
1.20
-
-
1.24
5
0.4
V
uA
V
PSON Control
Input Pull-up Current
High-Level Input Voltage
Low-Level Input Voltage
IPSON
VIHPS
VILPS
VPSON = 0V
- 150 -
2.4 -
-
- - 1.2
uA
V
V
Total Device
Supply Current
ICC VPSON = 5V
- - 1 mA
Switching Characteristics
Debounce Time(PSON)
Noise Debounce Time
PGO Delay Time(PGI to PGO)
tb1
tb2*
td1
25 38 51 ms
50 73 100 us
200 300 410 ms
Internal UVP Delay Time
PSON off to FPO Delay Time
td2
FPO goes low and
every time PGI > 1.2
51
75 102 ms
td4*
FPO goes low and
everytime PGI<1.2
200 300 410 ms
td3 tb1+1.6 tb1+2.3 tb1+3.2 ms
* : These parameters although guaranteed over the recommended operating conditions, are not 100% tested in production.
5
5 Page 
FAN7685/FAN7686/FAN7687
state after a debounce of 38ms. The FPO pin can be connected to +5V(or up to +15V) through a pull-up resistor.
Under Voltage Protection
The FAN7685/FAN7686/FAN7687 provide undervoltage protection(UVP) for the +3.3V, +5V, and +12V power rails. When
an undervoltage condition appears at one of the VS33(+3.3V), VS5(+5V), or VS12(+12V) input pins for more than 73us, the
PGO goes low and FPO output goes high. Also, this fault condition will be latched until the PSON is toggled from low to high
or the Vcc falls below a minimum operating voltage.
When the power supply is turned on by the AC input or PSON, an internal UVP delay time is 75ms. But at normal state an
UVP delay time is only a 73us debounce time. The need for undervoltage protection is often overlooked in off-line switching
power supply system design. But it is very important in battery powered or hand-held equipment since the TTL or CMOS
logic often malfunctions under UVP condition.
Over Voltage Protection(OVP)
The overvoltage protection(OVP) of the FAN7685/FAN7686/FAN7687 monitor +3.3V, +5V, and +12V. When an overvoltage
condition appears at one of the +3.3V, +5V, or +12V input pins for more than 73us, the FPO output goes high and the PGO
goes low. Also, this fault condition will be latched until the PSON is toggled from low to high or Vcc drops below a minimum
operating voltage. During overvoltage condition, most power supplies have the potential to deliver higher output voltages than
those normally specified or required. In unprotected equipment, it is possible for output voltages to be high enough to cause
internal or external damage to the system. To protect the system under these abnormal conditions, it is common practice to pro-
vide overvoltage protection within the power supply.
Because TTL and CMOS circuits are very vulnerable to overvoltage, it is becoming industry standard to provide overvoltage
protection on all +3.3V, +5V, and +12V outputs. Therefore, not only the +3.3V and +5V rails for the logic circuits on the moth-
erboard need to be protected, but also the +12V peripheral devices such as the hard disk, flopply disk, and CD-ROM players
etc., need to be protected.
Over Current Protection
In bridge or forward type, off-line switching power supplies, usually designed from medium to large power, the overload pro-
tection design needs to be very precise. Most of these types of power supplies are sensing the output current for an overload
condition. The trigger point needs to be set higher than the maximum load in order to prevent false turn-on.
During safety testing the power supply might have tied the output voltage direct to ground. If this happens during the nomal
operating, this is called a short-circuit or over current condition. When it happens before the power supply turns on, this is
called a short-circuit power supply turn-on. It can happen during the design period, in the production line, at quality control
inspection or at the end user. The FAN7685/FAN7686/FAN7687 provide an UVP and OCP with a 75ms delay after PSON is
set low.
The FAN7685/FAN7686/FAN7687 provide overcurrent protection(OCP) for the 3.3V, 5V, and 12V rails. When an overcurrent
condition appears at the OCP comparator input pins for more than 73us, the FPO output goes high and PGO goes low. Also,
this fault condition will be latched until PSON is toggled from low to high or Vcc is removed.
The resistor connected between the RI pin and the GND pin will introduce an accurate IO(RI) for the OCP function. Of course,
a more accurate resistor tolerance will be better. The formula for choosing the RI resistor is VRI/IO(RI). The IO(RI) range is
from 12.5uA to 62.5uA. Four OCP comparators and the IO(RI) section are supplied by VS12. Current drawn from the VS12pin
is less than 1mA.
Following is an example on calculating OCP for the 12V rail :
RI = I--O-V---(-R-R---I-I--) = 2--1--0-.-2--u--V-A--- = 60kΩ
IO(RI) × K × R(IS12)= R(sense) × I(OCP – Trip)
I(OCP – Trip)= 20u × 8 × 560Ω ⁄ (0.01Ω) = 9.2A
11
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet FAN7686.PDF ] | |
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