U6081B
TELEFUNKEN Semiconductors
Rev. A1, 14-Feb-97
1 (8)
PWM Power Control with Low Duty Cycle Switch Off
Description
U6081B is a PWM IC in bipolar technology for the
control of an N-channel power MOSFET used as a high
side switch. The IC is ideal for the use in the brightness
control (dimming) of lamps e.g., in dashboard
applications.
Features
D Pulse width modulation up to 2 kHz clock frequency
D Protection against short circuit, load dump overvol-
tage and reverse V
S
D Duty cycle 0 to 100%
D Output stage for power MOSFET
D Interference and damage protection according to
VDE 0839 and ISO/TR 7637/1.
D Ground wire breakage protection
D Charge pump noise suppressed
Ordering Information
Extended Type Number
Package
Remarks
U6081B
DIP8
Block Diagram
Current monitoring
+ short circuit detection
Output
Charge
pump
RC oscillator
Duty cycle
range
0/13 to 100 %
Control input
Voltage
monitoring
5
1
4
3
V
S
C
5
PWM
Logic
6
7
8
47 k
W
47 nF
R
sh
2
R
3
150
W
95 9752
V
Batt
C
3
C
2
C
1
Ground
Figure 1. Block diagram with external circuit
U6081B
TELEFUNKEN Semiconductors
Rev. A1, 14-Feb-97
2 (8)
Pin Description
1
2
3
4
5
6
7
8
95 9944
V
S
GND
V
I
Osc
Output
Sense
2 V
S
Delay
Pin
Symbol
Function
1
V
S
Supply voltage V
S
2
GND
IC ground
3
V
I
Control input (duty cycle)
4
Osc
Oscillator
5
Delay
Short circuit protection delay
6
Sense
Current sensing
7
2 V
S
Voltage doubler
8
Output
Output
Functional Description
Pin 1, Supply Voltage, V
s
or V
Batt
Overvoltage Detection
Stage 1:
If V
Batt
> 20 V occurs the external transistor will be
switched off and switched on again at V
Batt
< 18.5 V
(hysteresis).
Stage 2:
If V
Batt
> 28 V, the external transistor is switched on again
(load-dump protection). At the same time the voltage li-
mitation of the IC is reduced from V
S
26 V to V
S
20 V.
This leads to a hysteresis characteristic so that the load-
dump detection is switched off again only at V
Batt
< 23 V.
In this case the shortcircuit protection is not in operation.
Undervoltage Detection
In the event of voltages of approximately V
Batt
< 5.0 V,
the external FET is switched off and the latch for short-
circuit detection is reset.
A hysteresis ensures that the FET is switched on again at
approximately V
Batt
5.4 V.
Pin 2, GND
Ground-Wire Breakage
To protect the FET in the case of ground-wire breakage,
a 820-k
W resistor between gate and source is recom-
mended to provide proper switch-off conditions.
Pin 3, Control Input
The pulse width is controlled by means of an external
potentiometer (47 k
W). The characteristic (angle of
rotation/duty cycle) is linear. The duty cycle can be varied
from 0 to 100%. To avoid inadmissibly high filament cold
currents, the dimmer is switched off at duty cycles of
approximately < 10% or is switched on only at duty
cycles of approximately > 13% (hysteresis). It is possible
to further restrict the duty cycle with the resistors R
1
and
R
2
(see figure 2). Pin 3 is protected against short-circuit
to V
Batt
and ground GND (V
Batt
x
16.5 V).
Output Slope Control
The rise and fall time (t
r
, t
f
) of the lamp voltage can be
limited to reduce radio interference. This is done with an
integrator which controls a power MOSFET as source
follower. The slope time is controlled by an external
capacitor C4 and the oscillator current (see figure 2).
Calculation:
t
f
+ t
r
+ V
Batt
C
4
I
osc
With V
Batt
= 12 V, C
4
= 470 pF and I
osc
= 40
mA, we thus
obtain a controlled slope of
t
f
+ t
r
+ 12 V
470 pF
40
mA +
141
ms
A 100-
W resistor in series to C4 is recomended to damp
device oscillations (see figure 2).
Pin 4, Oscillator
The oscillator determines the frequency of the output
voltage. This is defined by an external capacitor, C
2
. It is
charged with a constant current, I, until the upper
switching threshold is reached. A second current source
is then activated which taps a double current, 2
I, from
the charging current. The capacitor, C
2
, is thus discharged
by the current, I, until the lower switching threshold is
reached. The second source is then switched off again and
the procedure starts again.
Example for oscillator frequency calculation:
V
T100
+ V
S
a
1
+ (V
Batt
* I
S
R
3
)
a
1
V
T
100
+ V
S
a
2
+ (V
Batt
* I
S
R
3
)
a
2
V
TL
+ V
S
a
3
+ (V
Batt
* I
S
R
3
)
a
3
where
U6081B
TELEFUNKEN Semiconductors
Rev. A1, 14-Feb-97
3 (8)
V
T100
+ High switching threshold (100% duty cycle)
V
T
t100
+ High switching threshold(
t
100% duty cycle)
V
TL
+ Low switching threshold
a
1
,
a
2
and
a
3
are fixed constant.
The above mentioned threshold voltages are calculated
for the following values given in the data sheet.
V
Batt
= 12 V, I
S
= 4 mA, R
3
= 150
W ,
a
1
= 0.7,
a
2
= 0.67 and
a
3
= 0.28.
V
T100
+ (12 V * 4 mA 150 W) 0.7 [ 8 V
V
T
t100
+ 11.4 V 0.67 + 7.6 V
V
TL
+ 11.4 V 0.28 + 3.2 V
For a duty cycle of 100%, an oscillator frequency, f, is as
follows:
f
+
I
osc
2
(V
T100
* V
TL
)
C
2
, whereas C
2
+ 22 nF
and I
osc
+ 40 mA
Therefore:
f
+
40
mA
2
(8 V * 3.2 V) 22 nF +
189 Hz
For a duty cycle of less than 100%, the oscillator
frequency, f, is as follows:
f
+
I
osc
2
(V
T
t100
* V
TL
)
C
2
) 4 V
Batt
C
4
whereas
C
4
= 470 pF
f
+
40
m
A
2
(7.6 V * 3.2 V) 22 nF ) 4 12 V 470 pF
+ 185 Hz
A selection of different values of C
2
and C
4
, provides a
range of oscillator frequency, f, from 10 to 2000 Hz.
Pins 5 and 6, Short-Circuit Protection and
Current Sensing
1. Short-Circuit Detection and Time Delay, t
d
The lamp current is monitored by means of an external
shunt resistor. If the lamp current exceeds the threshold
for the short-circuit detection circuit (V
T2
90 mV), the
duty cycle is switched over to 100% and the capacitor C
5
is charged by a current source of 20
m A (I
ch
I
dis
). The
external FET is switched off after the cut-off threshold
(V
T5
) is reached. Renewed switching on the FET is
possible only after a power-on reset. The current source,
I
dis,
ensures that the capacitor C
5
is not charged by
parasitic currents. The capacitor C
5
is discharged by I
dis
to typ. 0.7 V.
Time delay, t
d
, is as follows:
t
d
+ C
5
@ (V
T5
* 0.7 V) (I
ch
* I
dis
)
With C
5
= 330 nF and V
T5
= 9.8 V, (I
ch
I
dis
) = 20
mA,
we have
t
d
+ 330 nF @ (9.8 V * 0.7 V) 20 mA
+ 150 ms.
2. Current Limitation
The lamp current is limited by a control amplifier to pro-
tect the external power transistor. The voltage drop across
an external shunt resistor acts as the measured variable.
Current limitation takes place for a voltage drop of
V
T1
[100
mV. Owing to the difference
V
T1
V
T2
[
10 mV it is ensured that current limitation
occurs only when the short circuit detection circuit has
responded.
After a power-on reset, the output is inactive for an half
oscillator cycle. During this time, the supply voltage
capacitor can be charged so that the current limitation is
guaranteed in the event of a short circuit when the IC is
switched on for the first time.
Pins 7 and 8, Charge Pump and Output
Output, Pin 8, is suitable for controlling a power
MOSFET. During the active integration phase, the supply
current of the operational amplifier is mainly supplied by
the capacitor C
3
(bootstrapping). Additionally, a trickle
charge is generated by an integrated oscillator
(f
7
400 kHz) and a voltage doubler circuit. This
permits a gate voltage supply at a duty cycle of 100%.
U6081B
TELEFUNKEN Semiconductors
Rev. A1, 14-Feb-97
4 (8)
Absolute Maximum Ratings
Parameters
Symbol
Value
Unit
Junction temperature
T
j
150
C
Ambient temperature range
T
amb
40 to +110
C
Storage temperature range
T
stg
55 to +125
C
Thermal Resistance
Parameters
Symbol
Maximum
Unit
Junction ambient
R
thJA
120
K/W
Electrical Characteristics
T
amb
= 40 to +110
C, V
Batt
= 9 to 16.5 V, (basic function is guaranteed between 6.0 V to 9.0 V) reference point is
ground, unless otherwise specified (see figure 1). All other values refer to Pin GND (Pin 2).
Parameters
Test Conditions / Pins
Symbol
Min
Typ
Max
Unit
Current consumption
Pin 1
I
S
6.8
mA
Supply voltage
Overvoltage detection,
stage 1
V
Batt
25
V
Stabilized voltage
I
S
= 10 mA
Pin 1
V
Z
24.5
27.0
V
Battery undervoltage
detection
ON
OFF
V
Batt
4.4
4.8
5.0
5.4
5.6
6.0
V
Battery overvoltage detection Pin
2
Stage 1:
on
off
V
Batt
18.3
16.7
20.0
18.5
21.7
20.3
V
Stage 2:
on
off
V
Batt
25.5
19.5
28.5
23.0
32.5
26.5
V
Stabilized voltage
I
S
= 30 mA
Pin 1
V
Z
18.5
20.0
21.5
V
Short-circuit protection Pin 6
Short-circuit current
limitation
V
T1
= V
S
V
6
V
T1
85
100
120
mV
Short-circuit detection
V
T2
= V
S
V
6
V
T2
75
90
105
mV
T2
S
6
V
T1
V
T2
3
10
30
mV
Delay timer short-circuit detection, V
Batt
= 12 V Pin 5
Switched off threshold
V
T5
= V
S
V
5
V
T5
9.5
9.8
10.1
V
Charge current
I
ch
23
mA
Discharge current
I
dis
3
mA
Capacitance current
I
5
= I
ch
I
dis
I
5
13
20
27
mA
Voltage doubler
Pin 7
Voltage
Duty cycle 100%
V
7
2 V
S
Oscillator frequency
f
7
280
400
520
kHz
Internal voltage limitation
I
7
= 5 mA
V
7
26
27.5
30.0
V
g
(whichever is lower)
V
7
V
S+14
V
S+15
V
S+16
V
U6081B
TELEFUNKEN Semiconductors
Rev. A1, 14-Feb-97
5 (8)
Parameters
Test Conditions / Pins
Symbol
Min
Typ
Max
Unit
Switch-off at small duty cycles V
Batt
= 12 V Pin 3
Output disabled
V
3
/V
S
0.3
0.32
0.34
Output active
V
3
/V
S
0.32
0.34
0.36
Hysteresis switch-on
DV
3
/V
S
0.004
0.032
Gate output Pin 8
Voltage
Low level
V
8
0.35
0.70
0.95
V
g
V
Batt
= 16.5 V,
T
amb
= 110
C, R
3
= 150
W
8
1.5 *)
High level,
duty cycle 100%
V
8
V
7
Current
V
8
= Low level
I
8
1.0
mA
V
8
= High level, I
7
> | I
8
|
8
1.0
Oscillator
Frequency
Pin4
f
10
2000
Hz
Threshold cycle
Upper
V
8
+ High, a
1
+
V
T100
V
S
a
1
0.68
0.7
0.72
Upper
V
8
+ Low, a
2
+
V
T
t100
V
S
a
2
0.65
0.67
0.69
Lower
a
3
+
V
TL
V
S
a
3
0.26
0.28
0.3
Oscillator current
V
Batt
= 12 V
I
osc
34
45
54
mA
Frequency tolerance
C
4
open, C
2
= 470 nF,
duty cycle = 50%
f
6.0
9.9
13.5
Hz
*) Reference point is battery ground
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