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Número de pieza | UC3844B | |
Descripción | (UC3844B / UC3845B) HIGH PERFORMANCE CURRENT MODE CONTROLLERS | |
Fabricantes | ON Semiconductor | |
Logotipo | ||
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No Preview Available ! UC3844B, UC3845B,
UC2844B, UC2845B
High Performance
Current Mode Controllers
The UC3844B, UC3845B series are high performance fixed
frequency current mode controllers. They are specifically designed for
Off−Line and dc−dc converter applications offering the designer a
cost−effective solution with minimal external components. These
integrated circuits feature an oscillator, a temperature compensated
reference, high gain error amplifier, current sensing comparator, and a
high current totem pole output ideally suited for driving a power
MOSFET.
Also included are protective features consisting of input and
reference undervoltage lockouts each with hysteresis, cycle−by−cycle
current limiting, a latch for single pulse metering, and a flip−flop
which blanks the output off every other oscillator cycle, allowing
output deadtimes to be programmed from 50% to 70%.
These devices are available in an 8−pin dual−in−line and surface
mount (SOIC−8) plastic package as well as the 14−pin plastic surface
mount (SOIC−14). The SOIC−14 package has separate power and
ground pins for the totem pole output stage.
The UCX844B has UVLO thresholds of 16 V (on) and 10 V (off),
ideally suited for off−line converters. The UCX845B is tailored for
lower voltage applications having UVLO thresholds of 8.5 V (on) and
7.6 V (off).
Features
• Pb−Free Packages are Available
• Trimmed Oscillator for Precise Frequency Control
• Oscillator Frequency Guaranteed at 250 kHz
• Current Mode Operation to 500 kHz Output Switching Frequency
• Output Deadtime Adjustable from 50% to 70%
• Automatic Feed Forward Compensation
• Latching PWM for Cycle−By−Cycle Current Limiting
• Internally Trimmed Reference with Undervoltage Lockout
• High Current Totem Pole Output
• Undervoltage Lockout with Hysteresis
• Low Startup and Operating Current
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8
1
8
1
PDIP−8
N SUFFIX
CASE 626
SOIC−8
D1 SUFFIX
CASE 751
14
1
SOIC−14
D SUFFIX
CASE 751A
PIN CONNECTIONS
Compensation 1
Voltage Feedback 2
8 Vref
7 VCC
Current Sense 3
6 Output
RT/CT 4
5 GN
D
(Top View)
Compensation 1
NC 2
Voltage Feedback 3
NC 4
Current Sense 5
NC 6
RT/CT 7
14 Vref
13 NC
12 VCC
11 VC
10 Output
9 GND
8 Power Ground
(Top View)
ORDERING INFORMATION
See detailed ordering and shipping information in the package
dimensions section on page 2 of this data sheet.
DEVICE MARKING INFORMATION
See general marking information in the device marking
section on page 16 of this data sheet.
© Semiconductor Components Industries, LLC, 2004
September, 2004 − Rev. 3
1
Publication Order Number:
UC3844B/D
1 page UC3844B, UC3845B, UC2844B, UC2845B
ELECTRICAL CHARACTERISTICS (VCC = 15 V [Note 8], RT = 10 k, CT = 3.3 nF. For typical values TA = 25°C, for min/max values
TA is the operating ambient temperature range that applies [Note 9], unless otherwise noted.)
UC284XB
UC384XB, XBV
Characteristic
Symbol
Min Typ Max Min Typ Max Unit
PWM SECTION
Duty Cycle
Maximum (UC284XB, UC384XB)
Maximum (UC384XBV)
Minimum
TOTAL DEVICE
Power Supply Current
Startup (VCC = 6.5 V for UCX845B,
Startup (VCC = 14 V for UCX844B, BV)
Operating (Note 8)
DC(max)
DC(min)
ICC
%
47 48 50 47 48 50
− − − 46 48 50
−−0 − −0
mA
− 0.3 0.5 − 0.3 0.5
− 12 17 − 12 17
Power Supply Zener Voltage (ICC = 25 mA)
VZ 30 36 − 30 36
8. Adjust VCC above the Startup threshold before setting to 15 V.
9. Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible.
Tlow = 0°C for UC3844B, UC3845B
= − 25°C for UC2844B, UC2845B
Thigh = + 70°C for UC3844B, UC3845B
= + 85°C for UC2844B, UC2845B
= − 40°C for UC3844BV, UC3845BV
= +105°C for UC3844BV, UC3845BV
−
V
80 75
ÄÄÄÄÄÄÄÄÄÄ50
VCC = 15 V
TA = 25°C
1. CT = 10 nF
2. CT = 5.0 nF
3
70 3. CT = 2.0 nF
ÄÄÄÄÄ20
4. CT = 1.0 nF
5. CT = 500 pF
24
ÄÄÄÄÄ65 6. CT = 200 pF
1
ÄÄÄÄÄ8.0 7. CT = 100 pF
5.0 60
7
2.0
NOTE: Output switches at
1/2 the oscillator frequency
0.8
10 k 20 k
50 k
100 k
200 k
500 k
fOSC, OSCILLATOR FREQUENCY (kHz)
1.0 M
55
50
10 k
5
6
20 k
50 k 100 k 200 k
500 k 1.0 M
fOSC, OSCILLATOR FREQUENCY (kHz)
Figure 2. Timing Resistor
versus Oscillator Frequency
Figure 3. Output Deadtime
versus Oscillator Frequency
2.55 V
2.5 V
VCC = 15 V
AV = −1.0
TA = 25°C
3.0 V
2.5 V
VCC = 15 V
AV = −1.0
TA = 25°C
2.45 V
0.5 ms/DIV
Figure 4. Error Amp Small Signal
Transient Response
2.0 V
1.0 ms/DIV
Figure 5. Error Amp Large Signal
Transient Response
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5
5 Page UC3844B, UC3845B, UC2844B, UC2845B
VCC
7(12)
Vin
8(14)
4(7)
R2
2(3)
1(1)
R1
VClamp ≈
R
Bias
R
5.0V Ref
+
−
Osc
+
1.0 mA
VClamp
EA
2R
R
1.0V
T
S
Q
R
Comp/Latch
+
−
7(11)
6(10)
5(8)
Q1
3(5) RS
5(9)
1.67
ǒ ǓR2
ǒ ǓR1
)
1
+ 0.33x10−3
R1R2
R1 ) R2
Where: 0 ≤ VClamp ≤ 1.0 V
Ipk(max)
[
VClamp
RS
Figure 20. Adjustable Reduction of Clamp Level
8(14) R
Bias
5.0V Ref
R
+
−
Osc
4(7)
+
1.0mA
2(3)
1.0M EA
2R
R
1.0V
1(1)
C
tSoft−Start ≈ 3600C in mF
5(9)
T
S
Q
R
Figure 21. Soft−Start Circuit
VCC
7(12)
Vin
8(14) R
Bias
R
Osc
4(7)
+
1.0 mA
2(3)
R2
EA
1(1)
R1 MPSA63
2R
R
5.0V Ref
+
−
VClamp
1.0V
T
S
Q
R
Comp/Latch
5(9)
+
−
7(11)
6(10)
5(8)
3(5)
Q1
RS
1.67
ǒ ǓVClamp ≈
R2
R1
)
1
Where: 0 ≤ VClamp ≤ 1.0 V
ƪ ƫtSoft-Start + * In
1*
VC
3VClamp
C
R1 R2
R1 ) R2
Ipk(max)
[
VClamp
RS
VCC Vin
(12)
VPin 5
[
RS Ipk rDS(on)
rDM(on) ) RS
5.0V Ref
+
−
T
S
Q
R
+
−
If: SENSEFET = MTP10N10M
RS = 200
Then : VPin 5 [ 0.075 Ipk
(11)
G
(10)
D SENSEFET
S
K
M
(8)
Comp/Latch
(5) RS
1/4 W
Power Ground:
To Input Source
Return
Control Circuitry Ground:
To Pin (9)
Virtually lossless current sensing can be achieved with the implementation
of a SENSEFETt power switch. For proper operation during over−current
conditions, a reduction of the Ipk(max) clamp level must be implemented.
Refer to Figures 20 and 22.
Figure 22. Adjustable Buffered Reduction of
Clamp Level with Soft−Start
Figure 23. Current Sensing Power MOSFET
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11
11 Page |
Páginas | Total 20 Páginas | |
PDF Descargar | [ Datasheet UC3844B.PDF ] |
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