®
AN1596
-
APPLICATION NOTE
VIPower: HIGH SIDE DRIVERS FOR AUTOMOTIVE
V. Graziano - L. Guarrasi - A. Pavlin
INTRODUCTION
Today’s automotive market requires a continuous increasing of complexity and reliability in the electronic
systems. To achieve this, the concept of the automotive systems is more and more based on micro
controllers architecture driving integrated monolithic circuits that include a power stage, control, driving
and protection circuits on the same chip. Vertical Intelligent Power, a STMicroelectronics patented
technology, established over 13 years ago, uses a fabrication process which allows the integration of
complete digital and/or analog control circuits driving a vertical power transistor on the same chip. The
VIPower M0 technology used for making High Side Drivers (HSDs) produces a monolithic silicon chip,
which combines control and protection circuitry with a standard power MOSFET structure where the
power stage current flows vertically through the silicon (see figure 1).
Figure 1:
M0 chip structure
Driving circuitry
Enhancement and depletion NMOS
Power stage
VDMOS
p - well
n - type epilayer
n + substrate
Power stage output
The evolution of M0 technology made the drastic reduction of die size and of the resistance of devices
possible during conduction as well; each generation has seen a significant (from 40% to over 50%)
decrease in specific on-resistance and this translates into die size reduction, smaller packages, reduced
power dissipation and hence cost effective solutions. The third generation - the M0-3 - is in production
while STMicroelectronics is now developing the M0-4 and M0-5 technologies which will allow to achieve
less than 5mΩ R
DS(on)
in a PowerSO-10 package. High Side Drivers, with their integrated extra features
are power switches that can manage high currents and work up to about 36V supply voltage. They only
require a simple TTL logic input and incorporate a diagnostic output to the micro-controller. They can
drive an inductive load without the need for a freewheeling diode. For complete protection the devices
have an over-temperature sensing circuit that will shut the chip down under over-temperature conditions.
Due to the aggressive automotive environment, High Side Drivers are designed to work from -40°C to
+150°C. They also have an under-voltage shutdown feature. Each application exerts an external
November 2002
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AN1596 - APPLICATION NOTE
influence over the switch. A filament lamp or DC motor, for example, has in-rush currents that any switch
needs to handle. Solenoids and motors have an inductive effect and must lose the residual magnetism
when the current is turned off. External fault conditions can also stress the drivers and their associated
circuitry. The M0-3 High Side Driver can be divided in Analog and digital. This classification is done with
regard to diagnostic pin, which can be a two level signal pin or an analogue current sense pin. Diagnostic
information output helps the on Board microcontroller to quickly identify and isolate faults saving repair
time and often improving safety. High Side Drivers can reduce the size and weight of switch modules, and
where multiplexed systems are used, they dramatically reduce the size of the wiring harness.
Figure 2:
Generic HSD Internal Block Diagram
Vcc
Vcc
clamp
OVERVOLTAGE
UNDERVOLTAGE
Power
CLAMP
GND
Input
Status or
Current
sense
Isense =
I
OUT
/K
Logic
DRIVER
Current
LIMITER
OVERTEMPERATURE
OPENLOAD
ON STATE
OPENLOAD
OFF STATE
&
Vcc/OUT
SHORTED
OUT
STMicroelectronics HSDs are designed to provide the user with simple, self protected, remotely controlled
power switches. They have the general structure as shown in figure 2.
THE GENERAL FEATURES OF HIGH SIDE DRIVERS
Input
The 5V TTL input to these High Side Drivers is protected against electrostatic discharge (V
ESD
=4kV for
control pins and 5kV for Power pins). General rules concerning TTL logic should be applied to the input.
The input voltage is clamped internally at V
ICL
=6.8V as typical value. It is possible to drive the input with
a higher input voltage using an external resistor calculated to give a current not exceeding I
IN
=10mA (see
datasheets absolute maximum ratings section).
Internal power Supply
To accommodate the wide supply voltage range experienced by the logic and control functions, these
devices have an internal power supply. Some parts of the chip are only active when the input is high, the
charge pump for example. Therefore it is possible to conserve power when the device is idle. The new
M0-3 generation High Side Drivers supply current in the ON state is 5mA/channel. The internal power
consumption for the basic functions of the chip under any circumstances - even when the input is 0V - is
very low. The supply quiescent current I
S
, guaranteed at junction temperature of 25°C, a battery voltage
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AN1596 - APPLICATION NOTE
of 13V and the output pin grounded, is limited to a typical value of 10µA for a one channel HSD. In figure
3 a plot of typical I
S
values versus T
j
is shown for single channel and double channel monolithic HSDs.
Figure 3:
Stand-by current for single and double channel single chip HSDs Vs. junction temperature
Is(uA)
25
20
15
10
5
Double channel
Single Channel
0
-50
0
50
Tcase(ºC)
100
150
Thermal considerations
In order to choose the suitable HSD for a given load some important points must be highlighted. In the
worst-case operation (T
j
=150ºC), for a single channel HSD and in steady state conditions, the Joule effect
power developed by the device equals the Power dissipated according to the following equation:
2
R
DS
(
on
)
⋅
I
OUT
+
V
CC
⋅
I
S
=
T
J
−
T
amb
R
thj
−
amb
Assuming that the second term can be neglected, for a given load current I
OUT
a given package and heat
sink and a given ambient temperature (fixed at 85°C in automotive environment) the result is:
R
DS
(
on
)
(
150
°
C
)
=
T
J
−
T
amb
2
I
OUT
⋅
R
thj
−
amb
This is the maximum value of R
DS(on)
which can be chosen. The steady state on-resistance of HSDs is a
function of the junction temperature and in the datasheet its value is given at 25°C and this is
approximately doubled at 150°C. In some cases it may be convenient to use an HSD with a bigger R
DS(on)
in the same package. To still comply with the above equation we must reduce R
thi-amb
and have a better
heatsink. The trend from through-hole packages to low-cost SMD applications has led to think of the PCB
as a heatsink itself. In earlier packages (like PENTAWATT) a solid heatsink was either screwed or
clamped to the power package and it was easy to calculate the thermal resistance from the geometry of
the heatsink. In SMDs the heat path must be evaluated: chip (junction) - leadframe - case or pin - footprint
- PCB materials - PCB volume - surroundings. To evaluate static thermal properties of an SMD an
associated static equivalent circuit (see figure 4) can be considered. The power dissipation of the chip is
symbolized by a current source whilst the ambient temperature is represented by a voltage source. By
estimating the PCB heatsink area in a real application, the user can easily determine R
thj-amb
in still air,
3/24
AN1596 - APPLICATION NOTE
which is the worst case; in real applications the values for the heat resistance are much lower. The
following equation applies:
R
thj
−
amb
=
Figure 4:
Static thermal equivalent circuit
T
j
−
T
amb
P
V
R
thj-amb
Die
Die Bond
Lead-frame
Solder
Heatsink
P
d
T
j
R
thj-case
T
case
R
thcase-amb
T
amb
In the above equation, the power loss P
V
and the ambient temperature T
amb
can be easily determined in
a temperature chamber. The chip temperature T
j
can be derived during the operation, measuring the
device’s R
DS(on)
= (V
CC
- V
OUT
)/I
OUT
.
Figure 5:
PowerSO-10 recommended layout for high power dissipation capability
R
thjamb
= 50 C/W
recomended pad layout
R
thjamb
= 35 C/W
pad layout + 6 cm2 on board heat sink
R
thjamb
= 20 C/W
pad layout + ground layers
R
thjamb
= 15 C/W
pad layout + ground layers + 16 via holes
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AN1596 - APPLICATION NOTE
Having the characteristic R
DS(on)
versus T
j
, the relevant chip temperature can be derived. Figure 5 shows
different PCB layout for PowerSO-10 package. The thermal resistance R
thj-amb
can be reduced from
50°C/W to 15°C/W by holes linking different copper layers.
In the VIPower HSDs datasheets there are two sections concerning the thermal management. The first
one shows the thermal calculation in order to find out the junction temperature in static conditions together
with a plot of thermal resistance junction to ambient versus PCB heatsink area. The second one shows a
plot of thermal impedance junction ambient in single pulse and the thermal model is shown with relevant
thermal resistances and capacitors values (easy simulations can be performed both in static conditions
and during transients as, for example, switching on a load with high in rush currents or PWM operation).
In figure 6 an example of a double channel HSD thermal model is shown.
Figure 6:
VND830 (SO16L package) thermal model
Area/island (cm
2
)
R1 (°C/W)
R2 (°C/W)
R3 (°C/W)
R4 (°C/W)
R5 (°C/W)
R6 (°C/W)
C1 (W.s/°C)
C2 (W.s/°C)
C3 (W.s/°C)
C4 (W.s/°C)
C5 (W.s/°C)
C6 (W.s/°C)
Footprint
0.15
0.8
2.2
12
15
37
0.0006
2.10E-03
1.50E-02
0.14
1
3
6
Tj_1
Pd1
C1
C2
C3
C4
C5
C6
R1
R2
R3
R4
R5
R6
Tj_2
22
C1
C2
R1
Pd2
R2
T_amb
5
THE CONTROL AND PROTECTION CIRCUIT
Protection against low energy spikes and load dump
The voltage transients are very dangerous hazards to the automotive electronics. The transients tend to
be either low energy- high voltage spikes or high energy-high voltage, up to 125V levels. The low energy
spikes are generated by fast turnoff of high-current inductive loads, such as air-conditioning compressor
clutches. This effect, combined with inductive behavior of wires, causes an overshoot voltage on the
devices V
CC
pin. M0-3 High Side Drivers have an internal protection designed to clamp the low energy
spikes to 41V (V
CC
clamp block in figure1). In this situation the energy can flow through the internal
MOSFET T2 that is turned on through an internal clamp circuit (see figure 7).
M0-3 High Side Drivers are designed to successfully pass the 1, 2, 3a, 3b and 4 ISO-7637 standard
pulses test (see table 1 carried in HSDs datasheets as well) - simulating the low energy voltage spikes.
These values must be added to the voltage battery (for cars about 13.5V) to obtain the actual voltage. The
N.5 ISO7637 pulse simulates the alternator load dump in the case of a Generator with an internal
impedance of 2Ω and different values of magnetic field of the excitation circuit (see figure 8 for the level
IV pulse); this occurs when the battery is disconnected whilst being charged by the alternator. The voltage
spike can reach duration of approximately ½ second and it is of high-energy nature because of the
alternator's low source impedance. Where a centralized clamp circuit is not provided or ISO7637 rated
devices are not used, an external zener D
ld
diode is necessary to clamp the transient voltage battery (see
figure 7). This is done because an internal protection against load dump would require a larger die size
and - therefore - higher cost than putting on a module level protection.
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