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High-Side Switch for Motors and Lamps Power Electronics Technology September 2004
www.powerelectronics.com
22
High-Side Switch for
Motors and Lamps
By Kandarp Pandya, Senior Staff Applications Engineer,
Vishay Siliconix, Santa Clara, Calif.
L
On-chip protection enables startup of a motor
under heavy load conditions or a cold filament
lamp while preventing damage to control
circuitry under locked-rotor and short-circuit
conditions.
ment them are
often intricate and
elaborate.
Most motor-
control designs
consist of control
i n t e l l i g e n c e ,
implemented with
discrete compo-
nents or an IC
controller, and a
MOSFET, which
serves as the
power-handling
device. The chal-
lenge is to protect
the MOSFET un-
der fault condi-
tions. This can be
done using several
discrete compo-
nents or with an integrated solution, such as a protected
high-side smart MOSFET switch.
Fig. 1 shows a 12-V, 2-A unidirectional motor control
using a fully protected, high-side MOSFET switch,
the Si4750DY from Vishay Siliconix. Packaged in the SMT
SO-8 package, this smart MOSFET combines a
50-m power MOSFET with a controller IC. This device
takes the logic-level signal and provides a high-side gate
drive for the MOSFET, and also protects the MOSFET by
removing the gate drive under fault conditions.
The Si4750DY is powered by a 12-Vdc-regulated
supply (pins 2, 3 and 4 to +12 Vdc and pin 7 to ground).
On-board capacitor filters (C1 and C2) ensure stable op-
eration. For C1, a 100-µF aluminum electrolytic capacitor
provides on-board charge to absorb minor fluctuations in
GND
V_bat
C2
0.1 mfd, 25 v
C1
100 mfd, 25 v
+
U1
Si4750DY
LI
4.7 uH
C3
1 MFD,
25 V
R3
100 E
PGND
Output
Load
12 V / 2 A
5
6
S
S
V_bat
2
3
4
V_bat
V_st
In
7
8
1
2
1
Z1
5.1 V
JP1
Jumper
R2
100 k
R1
1.2 k
Controller
IC
Fig. 1.
A fully protected, high-side MOSFET switch, the Si4750DY,
implements a 12-V, 2-A unidirectional motor control circuit.
ow-power dc motor control is one of
the most common applications for
power electronics, and one for which
low-cost solutions are in demand. De-
sign engineers are always on the look-
out for solutions that cater to the need for low costs and
short design cycles. Even so, such applications have their
subtle aspects and warrant a careful design.
The principal requirement for the electronics used
in motor control applications is the ability to start the
motor under heavy load conditions while preventing
damage to control circuitry under locked-rotor and
short-circuit conditions. Although these requirements
are easy to understand, the electronics required to imple-
Fig. 2.
Measurements taken on the motor
control circuit in Fig. 1 include V
IN
, a 5-V logic
level control signal (Ch1); V
OUT
, a 12-V output
signal measured at the motor terminal (Ch2);
and I
L
, the motor current (Ch3, 0.5 A/div). At
startup, the motor draws a peak pulse of 5.25 A,
which falls to a nominal current of 0.5 A.
Tek
Stop:
Single Seq 5.00 kS/s
[ T ]
1
2
3
T
T
T
Ch 3
Ch1 5.00 V
10.0 mVCh1 5.00 V M 10.0 ms Ch1
5.1 V Power Electronics Technology September 2004
www.powerelectronics.com
24
HIGH-SIDE SWITCH
In
t
t
off(SC)
t
rm
I
L(SCp)
I
L(SCr)
t
Fig. 3.
A functional block diagram of Si4750DY reveals MOSFET-
protection features.
Fig. 4.
The Si4750DY controls turn-on into a cold
filament lamp load, producing the load current
waveform shown.
the battery supply. For C2, a 0.1-µF
ceramic capacitor filters any high-
frequency noise present on the line.
A 5-Vdc Zener regulated logic-
level supply (resistor R1 and Zener
diode Z1) provides the drive signal
and biases the status pin through an
external pull-up resistor (R2). Setting
resistor R1 to 1.2 k provides a
Zener current of 5.8 mA {(V
bat
-V
Zener
)/
R1 = (12 V - 5.1 V)/1.2 k = 5.83 mA},
which is a little more than 5% of the
500-mW Zener capacity but adequate
to stabilize the Zener operation. A
100-k pull-up resistor for R2 con-
nects the status pin 8 to the 5-V sup-
ply with negligible power loss.
The output of the device drives a
load, which in this case is a dc motor.
A dummy load (resistor R3 and ca-
pacitor C3) avoids a floating output
condition. The value of resistor R3 is
chosen to provide approximately 6%
of the rated 2-A load (R3 = V
OUT
/120
mA = 12 V/120 mA = 100 ). The
value of capacitor C3 at 1 µF is a typi-
cal capacitive load. The inductor (L1)
has a special purpose and function
during short-circuit mode that well
discuss later. A jumper (JP1) between
5 V and the input pin 1 [IN] controls
the circuit operation.
Circuit operation can be divided
into two states: normal operation with
the rated load of 2 A at 12 V, and heavy
load/short-circuit operation.
In the first state, normal operation
up to the rated load, a high logic-level
signal on pin 1 of Si4750DY turns on
the output and powers the motor,
while a low logic-level signal on pin 1
of Si4750DY turns off the output
and de-energizes the motor. Fig. 2
shows a typical wave form for V
IN
, a
5-V logic level in-
put; V
OUT
, a 12-V
output on the mo-
tor terminal; and
I
L
, the motor
current.
In the second
state, heavy load/
short-circuit op-
eration, the self-
limiting current is
set to 20 A. This
mode of operation uses one or more
protection features of the device to
protect the MOSFET. Fig. 3 shows all
of these in functional block diagram
for the Si4750DY device.
In the case where the load is a fila-
ment lamp, the initial condition is a
cold filament condition, which is clas-
sified as a fault condition. However,
these conditions are inevitable, espe-
cially in the automotive industry. The
resistance of a filament lamp at
power-on is very low and the current
draw is almost like a short-circuit con-
dition. Typical overcurrent protection
circuitry instantaneously shuts down
the power to protect the power device.
As a result, the circuit is unable to start
the lamp load.
The Si4750DY has a novel built-in
feature to circumvent the problem of
cold filament start-up for automotive
lamp loads (Fig. 4). Here, the over-
current tripping feature (set at the
I
L(SCp)
level in Fig. 4) is disabled for
first 400 µs (shown as t
off(SC)
in Fig. 4),
but supervised by thermal shutdown.
I
L(SCp)
represents the initial peak short
circuit current, while I
L(SCr)
represents
the repetitive short-circuit current
limit. This limit is lower than initial
peak due to increased operating junc-
tion temperature of the MOSFET.
The latter leads to higher R
ds(on)
and
in turn lowers the short-circuit cur-
rent limit.
This MOSFET is turned off only
when the junction temperature ex-
ceeds the 150°C rating. At power-on
into the cold filament lamp, the cur-
rent shoots up instantaneously, heat-
ing up the lamp filament. By the time
thermal shutdown occurs, the fila-
ment is heated up enough and
offers higher resistance at next cycle
to ease the current draw and permit
the full turn on of the lamp. This fea-
ture also helps the motor to start up
under heavy load conditions.
During motor startup under heavy
load conditions, either over-current
or thermal shutdown initiates the
tripping. Which of these comes into
play depends on the operating ambi-
ent and resulting junction tempera-
ture. Because the circuit operates in
pulse-by-pulse tripping mode, it re-
sets at a much faster rate at lower am-
bient temperatures.
In other words, the device on-
period (torque buildup period) is
longer than the device off-period (no
torque period). In turn, this permits
torque to build up and provides a
higher starting torque. Hence, the
motor has a better chance to overcome
a heavy load condition and start run-
Tek
Stop:
Single Seq 25.0 kS/s
[ T ]
3
T
Ch 3
10.0 mVW
M 2.00 ms Ch3
9.4 mV
Fig. 5.
In overload, a 50-W motor is unable to
start due to thermal shutdown even before
current limit is reached. Load currents
measured on Ch3 are shown on a 5-A/div scale.
V
bat
V
st
In
Load
S
Load
source
Sense
diode
MOSFET die
Charge pump
and gate drive
Overload
and short
circuit
Controller
Voltage
regulator
ESD
Controller IC
ESD
Thermal shutdown
with hysterisis Power Electronics Technology September 2004
www.powerelectronics.com
26
HIGH-SIDE SWITCH
Tek
Stop:
Single Seq 25.0 kS/s
[ T ]
3
T
Ch 3
5.00 VW
10.0 mVW
M 2.00 ms Ch3
9.4 mV
Ch1
1
T
Fig. 6.
Pulse-by-pulse current limiting protects
the Si4750s MOSFET under short-circuit
conditions. Measurements of load curr