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Электронный компонент: U6084B-FP

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Rev. 4677BAUTO02/04
Features
Pulse-width Modulation up to 2 kHz Clock Frequency
Protection against Short-circuit, Load-dump Overvoltage and Reverse V
S
Duty-cycle 0 to 100% Continuously
Output Stage for Power MOSFET
Interference and Damage Protection According to VDE 0839 and ISO/TR 7637/1
Charge-pump Noise Suppressed
Ground-wire Breakage Protection
Description
The U6084B is a PWM-IC with bipolar technology designed for the control of an
N-channel power MOSFET used as a high-side switch. The IC is ideal for use in the
brightness control (dimming) of lamps such as in dashboard applications. For constant
brightness, the preselected duty-cycle can be reduced automatically as a function of
the supply voltage.
Figure 1. Block Diagram with External Circuit
Current monitoring
+ short circuit detection
Output
Charge
pump
RC oscillator
Duty cycle
range
0-100%
Duty cycle
reduction
Control input
Short circuit
latch monitoring
Voltage
monitoring
Enable/
disable
9
11
16
6
3
4
PWM
Logic
C
1
12
13
14
47 nF
1
150
2
5
47 k
Ground
C
2
C
5
C
3
R
3
V
S
V
Batt
R
sh
PWM Power
Control with
Automatic
Duty-cycle
Reduction
U6084B
2
U6084B
4677BAUTO02/04
Pin Configuration
Figure 2. Pinning
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
EN/DIS
VI
REDUCT
NC
OSC
NC
NC
VS
NC
OUTPUT
2 VS
SENSE
DELAY
NC
LATCH
Pin Description
Pin
Symbol
Function
1
GND
IC ground
2
EN/DIS
Enable/disable
3
VI
Control input (duty cycle)
4
REDUCT
Duty cycle reduction
5
NC
Attenuation
6
OSC
Oscillator
7
NC
Not connected
8
NC
Not connected
9
LATCH
Status short-circuit latch
10
NC
Not connected
11
DELAY
Short-circuit protection delay
12
SENSE
Current sensing
13
2VS
Voltage doubler
14
OUTPUT
Output
15
NC
Not connected
16
VS
Supply voltage V
S
3
U6084B
4677BAUTO02/04
Functional Description
Pin1 GND
Ground-wire Breakage
To protect the FET in case of ground-wire breakage, a 820 k
resistor between gate
and source is recommended to provide proper switch-off conditions.
Pin 2 Enable/Disable
The dimmer can be switched on or off, with pin 2, independently of the set duty cycle.
Pin 3 Control Input
The pulse width is controlled by means of an external potentiometer (47 k
). The char-
acteristic (angle of rotation/duty cycle) is linear. The duty cycle be varied from 0 to
100%. It is possible to further restrict the duty cycle with resistors R
1
and R
2
(see Figure 3 on page 8).
Pin 3 is protected against short-circuit to V
Batt
and ground GND (V
Batt
16.5 V).
Pin 4 Duty Cycle
Reduction
With pin 4 connected according to Figure 3 on page 8, the set duty cycle is reduced to
V
Batt
12.5 V. This causes a power reduction in the FET and in the lamps. In addition,
the brightness of the lamps is largely independent of the supply voltage range,
V
Batt
= 12.5 to 16 V.
Output Slope Control
The rise and fall time (t
r
, t
f
) of the lamp voltage can be limited to reduce radio interfer-
ence. This is done with an integrator which controls a power MOSFET as source
follower. The slope time is controlled by an external capacitor C
4
and the oscillator cur-
rent (see Figure 3 on page 8).
Calculation:
With V
Batt
= 12 V, C
4
= 470 pF and I
osc
= 40 A, we thus obtain a controlled slope of
Pin 5 Attenuation
Capacitor C
4
connected to pin 5 damps oscillation tendencies.
Pin 6 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 cur-
rent, 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 once more.
Table 1. Pin 2 Function
V
2
Function
Approximately > 0.7 V or open
Disable
< 0.7 V or connected to pin 1
Enable
t
f
t
r
V
Batt
C
4
I
osc
---------
=
=
t
f
t
r
12 V
470 pF
40 A
------------------
141 s
=
=
4
U6084B
4677BAUTO02/04
Example for Oscillator
Frequency Calculation
where
V
T100
= High switching threshold 100% duty cycle
V
T<100
= High switching threshold < 100% duty cycle
V
TL
= Low switching threshold
1
,
2
and
3
are fixed values
The above mentioned threshold voltages are calculated for the following values given in
the datasheet.
V
Batt
= 12 V, I
S
= 4 mA, R
3
= 150
,
1
= 0.7,
2
= 0.67 and
3
= 0.28.
V
T100
= (12 V - 4 mA
150
)
0.7
8 V
V
T<100
= 11.4 V
0.67 = 7.6 V
V
TL
= 11.4 V
0.28 = 3.2 V
For a duty cycle of 100%, the oscillator frequency, f, is as follows:
where C
2
= 22 nF and I
osc
= 40 A
Therefore:
For a duty cycle of less than 100%, the oscillator frequency, f, is as follows:
where C
4
= 470 pF
A selection of different values of C
2
and C
4
provides a range of oscillator frequencies
from 10 to 2000 Hz.
Pins 7, 8, 10 and 15
Not connected.
Pin 9 Status Short
Circuit Latch
The status of the short-circuit latch can be monitored via pin 9 (open collector output).
V
T100
V
S
1
V
Batt
I
S
R
3
(
)
1
=
=
V
T<100
V
S
2
V
Batt
I
S
R
3
(
)
2
=
=
V
TL
V
S
3
V
Batt
I
S
R
3
(
)
3
=
=
f
I
osc
2
V
T100
V
TL
(
)
C
2
------------------------------------------------------------
=
f
40 A
2
8 V
3.2 V
(
)
22 nF
-------------------------------------------------------------------
189 Hz
=
=
f
I
osc
2
V
T<100
V
TL
(
)
C
2
4
+
V
Batt
C
4
-----------------------------------------------------------------------------------------------------------
=
f
40 A
2
7.6 V
3.2 V
(
)
22 nF
4
+
12 V
470 pF
-------------------------------------------------------------------------------------------------------------------------------
185 Hz
=
=
Table 2. Pin 9 Function
Pin 9
Function
L
Short-circuit detected
H
Not short-circuit detected
5
U6084B
4677BAUTO02/04
Pins 11 and 12 Short-circuit Protection and Current Sensing
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 capacitor C
5
is charged by a current source of 20 A
(I
ch
- I
dis
). The external FET is switched off after the cut-off threshold (V
T11
) is reached.
Renewed switching on the FET is possible only after a power-on reset. The current
source, I
dis
, ensures that capacitor C
5
is not charged by parasitic currents. Capacitor C
5
is discharged by I
dis
to typ. 0.7 V.
Time delay, t
d
, is as follows:
With C
5
= 330 nF and V
Batt
= 12 V, we have
Current Limitation
The lamp current is limited by a control amplifier that protects the external power transis-
tor. 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 differ-
ence V
T
- V
T2
10 mV, current limitation occurs only when the short-circuit detection
circuit has responded.
After a power-on reset, the output is inactive for half an oscillator cycle. During this time,
the supply voltage capacitor can be charged so that current limitation is guaranteed in
the event of a short-circuit when the IC is switched on for the first time.
Pins 13 and 14 Charge
Pump and Output
Pin 14 (output) is suitable for controlling a power MOSFET. During the active integration
phase, the supply current of the operational amplifier is mainly supplied by capacitor C
3
(bootstrapping). Additionally, a trickle charge is generated by an integrated oscillator
(f
13
400 kHz) and a voltage doubler circuit. This permits a gate voltage supply at a
duty cycle of 100%.
Pin 16 Supply Voltage,
V
s
or V
Batt
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.
Overvoltage Detection
Stage 1
If overvoltages of V
Batt
> 20 V (typically) occur, the external transistor is switched off and
switched on again at V
Batt
< 18.5 V (hysteresis).
Stage 2
If V
Batt
> 28.5 V (typically), the voltage limitation of the IC is reduced from 26 V to 20 V.
The gate of the external transistor remains at the potential of the IC ground, thus pro-
ducing voltage sharing between the FET and lamps in the event of overvoltage pulses
(e.g., load-dump). The short-circuit protection is not in operation. At V
Batt
< 23 V, the
overvoltage detection stage 2 is switched off.
t
d
C
5
V
11
0.7 V
(
)
I
ch
I
dis
(
)
----------------------------------
=
t
d
330 nF
9.8 V
0.7 V
(
)
20
A
--------------------------------------
150 ms
=
=