TC649 Datasheet, PDF (15/28 Pages) Microchip Technology – PWM Fan Speed Controller with Auto-Shutdown and FanSense™ Technology

TC649
5.6 Latch-up Considerations
As with any CMOS IC, the potential exists for latch-up
if signals are applied to the device which are outside
the power supply range. This is of particular concern
during power-up if the external circuitry (such as the
sensor network, V_AS divider or shutdown circuit) are
powered by a supply different from that of the TC649.
Care should be taken to ensure that the TC649’s V_DD
supply powers up first. If possible, the networks
attached to V_IN and V_AS should connect to the V_DD sup-
ply at the same physical location as the IC itself. Even
if the IC and any external networks are powered by the
same supply, physical separation of the connecting
points can result in enough parasitic capacitance and/
or inductance in the power supply connections to delay
one power supply “routing” versus another.
5.7 Power Supply Routing and
Bypassing
Noise present on the V_IN and V_AS inputs may cause
erroneous operation of the FAULT output. As a result,
these inputs should be bypassed with a 0.01 µF
capacitor mounted as close to the package as possible.
This is especially true of V_IN, which is usually drive from
+5V
a high impedance source (such as a thermistor). Addi-
tionally, the V_DD input should be bypassed with a 1 µF
capacitor with grounds being kept as short as possible.
To keep fan noise off the TC649 ground pin, individual
ground returns for the TC649 and the low side of the
fan current sense resistor should be used.
Design Example
Step 1. Calculate R₁ and R₂ based on using an NTC
having a resistance of 10 kΩ at T_MIN (25°C)
and 4.65 kΩ at T_MAX (45°C) (see Figure 5-9).
R₁ = 20.5 kΩ
R₂ = 3.83 kΩ
Step 2. Set auto-shutdown Level.
V_AS = 1.8V.
Limit the divider current to 100 µA
R₅ = 33 kΩ
R₆ = 18 kΩ
Step 3. Design the output circuit.
Maximum fan motor current = 250 mA.
Q₁ beta is chosen at 50 from which
R₇ = 800 Ω.
RESET
Shutdown
Open-
Drain
Device
(Optional)
R1
20.5 kΩ
R2
3.83 kΩ
+5V
NTC
CB
10 kΩ
1 µF
@ 25˚C
1
V_IN
8
V_DD
C_B
0.01 µF
4
GND
6
FAULT
+5V
R5
33 kΩ
3 V_AS
C_B
0.01 µF
R6
18 kΩ
2 _CF
TC649
V_OUT ⁷
SENSE ⁵
C1
1 µF
Fan Fault
C_SENSE
0.1 µF
+12V
Fan
Q1
R7
800 Ω
R_SENSE
2.2 Ω
FIGURE 5-9:
Design Example.
 2002 Microchip Technology Inc.
DS21449C-page 15