UC3846-1.pdf

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UC3846
SMPS Controller
Features
• Automatic Feed Forward Compensation
• Programmable Pulse by Pulse Current Limiting
• Automatic Symmetry Correction in Push-Pull
Configuration
• Enhanced Load Response Characteristics
• Parallel Operation Capability for Modulator Power
Systems
• Differential Current Sense Amplifier with Common Mode
Range
• Double Pulse Suppression
• 200mA Totem-Pole Outputs
•
±2%
Band gap Reference
• Under-Voltage Lockout
• Soft-Start Capability
• Shutdown Terminal
• 500KHz Operation
Description
The UC3846 control IC provides all of the necessary fea-
tures to implement fixed frequency, current mode control
schemes while maintaining a minimum external parts count.
The superior performance of this technique can be measured
in improved line regulation, enhanced load response char-
acteristics, and a simpler, easier-to-design control loop.
Topological advantages include inherent pulse-by-pulse cur-
rent limiting capability, automatic symmetry correction for
push-pull converters, and the ability to parallel “power
module
,,
while maintaining equal current sharing. Protection
circuitry includes built-in-under-voltage lockout and pro-
grammable current limit in addition to soft-start capability. A
shutdown function is also available which can initiate either
a complete shutdown with automatic restart or latch the
supply off. Other features include fully latched operation,
double pulse suppression, deadtime adjust capability, and
±2% trimmed bandgap reference. The UC3846 features low
outputs in the OFF state.
16-DIP
1
Internal Block Diagram
10
Rev. 5.0
©2000 Fairchild Semiconductor International
UC3846
Absolute Maximum Ratings
Parameter
Supply Voltage
Collector Supply Voltage
Output Current, Sink of Source (Peak)
Reference Output Current
Soft Start Sink Current
Sync Output Current
Error Amplifier Output Current
Oscillator Changing Current
Power Dissipation (T
A
= 25°C)
Operating Temperature
Storage Temperature
Lead Temperature (Soldering, 10sec)
Symbol
V
CC
V
C
I
O
I
REF
I
SINK(S.S)
I
SYNC
I
O(E.A)
I
CHG(OSC)
P
D
T
OPR
T
STG
T
LEAD
Value
40
40
500
30
50
5
5
5
1000
0 ~ +70
-65 ~ +150
+300
Unit
V
V
mA
mA
mA
mA
mA
mA
mW
°C
°C
°C
Electrical Characteristics
(V
CC
=15V, T
A
=0°C to +70°C, unless otherwise specified)
Parameter
REFERENCE SECTION
Reference Output Voltage
Line Regulation
Load Regulation
Temperature Stability(Note 6)
Output Voltage Range (Note 6)
Short Circuit Output Current
Output Noise Voltage(Note 6)
Long-Term Stability(Note 6)
V
REF
∆V
REF
∆V
REF
ST
T
V
REF
I
SC
V
NO
S
T
V
REF
= 0V
f = 10Hz to 10KHz, T
J
= 25°C
T
J
= 125°C, 1KHz
T
J
= 25°C, I
REF
= 1mA
V
CC
= 8 to 40V
I
REF
1 to 10mA
-
Line,Load,Temp
5.00
-
-
-
4.95
-10
-
2
5.10
5
3
0.4
-
-45
100
5
5.20
20
15
1.0
5.25
-
-
8
V
mV
mV
mV/°C
V
mA
uV
mV
Symbol
Conditions
Min.
Typ.
Max.
Unit
2
UC3846
Electrical Characteristics
(V
CC
= 15V,T
A
=0°C to +70°C, unless otherwise specified)
Parameter
OSCILLATOR SECTION (Note 2)
Initial Accuracy
Frequency Change with Voltage
Frequency Change with
Temperature (Note 6)
Sync Output High Level
Sync Output Low Level
Sync Input High Level
Sync Input Low Level
Sync Input Current
ERROR AMPLIFIER SECTION
Input Offset Voltage
Input Bias Current
Input Offset Current
Common-Mode Range
Open Loop Voltage Gain
Unity Gain Bandwidth(Note 6)
Common Mode Rejection Ratio
Power Supply Rejection Ratio
Output Sink Current
Output Source Current
High Output Voltage
Low Output Voltage
Amplifier Gain (Note 1, 3)
Maximum Differential Input
Signal (V
4
- V
3
) (Note 1)
Input Offset Voltage (Note 1)
Common Mode Rejection Ratio
Power Supply Rejection Ratio
Input Bias Current (Note 1)
Input Offset Current (Note 1)
Delay to Outputs (Note 6)
V
IO
I
BIAS
I
IO
V
CM
G
VO
BW
CMRR
PSRR
I
SINK
I
SOURCE
V
OH
V
OL
G
V
V
I(DIFF,MAX)
V
IO
CMRR
PSRR
I
BIAS
I
IO
t
D
-
-
-
V
CC
= 8 to 40V
V
O
= 1.2 to 3V, V
CM
= 2V
T
J
= 25°C
V
CM
= 0 to 38V, V
CC
= 40V
V
CC
= 8 to 40V
V
IO
= -15mV to 5V, V
7
= 2.5V
R
L
= 15KΩ
R
L
= 15KΩ
-
V
3
= 0V, Pin 1 open
R
L
= 15KΩ, Pin 1 open
V
1
= 0.5V, Pin 1 open
V
CM
= 1 to 12V
V
CC
= 8 to 40V
V
1
= 0.5V, Pin 7 open
V
1
= 0.5V, Pin 7 open
T
J
= 25°C
-
-
-
0
80
0.7
75
80
2
-0.4
4.3
-
2.5
1.1
-
60
60
-
-
-
0.5
-0.6
40
-
105
1.0
100
105
6
-0.5
4.6
0.7
2.75
1.2
5
83
84
-2.5
0.08
200
5
-1
250
V
CC
2
-
-
-
-
-
-
-
1
3.0
-
25
-
-
-10
1
500
mV
uA
uA
V
dB
MHz
dB
dB
mA
mA
V
V
V
V
mV
dB
dB
uA
uA
ns
ACCUR
∆f/∆V
CC
∆f/∆T
V
OH(SYNC)
V
OL(SYNC)
V
IH(SYNC)
V
IL(SYNC)
I
I(SYNC)
V
8
= 0V
V
8
= 0V
Sync Voltage = 3.9V, V
8
= 0V
T
J
= 25°C
V
CC
= 8 to 40V
-
-
-
39
-
-
3.9
-
3.9
-
-
43
1
1
4.35
2.3
-
-
1.3
47
2
-
-
2.5
-
2.5
1.5
KHz
%
%
V
V
V
V
mA
Symbol
Conditions
Min.
Typ.
Max.
Unit
CURRENT SENSE AMPLIFIER SECTION
3
UC3846
Electrical Characteristics
(V
CC
=15V, T
A
=0°C to + 70°C, unless otherwise specified)
Parameter
CURRENT LIMIT ADJUST SECTION
Current Limit Offset Voltage (Note 1)
Input Bias Current
SHUTDOWN TERMINAL SECTION
Threshold Voltage
Input Voltage Range
Minimum Latching Current (Note 4)
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold
Threshold Hysteresis
OUTPUT SECTION
Collector-Emitter Voltage
Collector Leakage Current
Low Output Voltage 1
Low Output Voltage 2
High Output Voltage 1
High Output Voltage 2
Rise Time (Note 6)
Fall Time (Note 6)
TOTAL STANDBY CURRENT
Supply Current
I
CC
-
-
17
21
mA
V
CEO
I
LEAK
V
OL
1
V
OL
2
V
OH
1
V
OH
2
t
R
t
F
V
C
= 40V
I
SINK
= 20mA
I
SINK
= 100mA
I
SOURCE
= 20mA
I
SOURCE
= 100mA
C
L
= 1nF, T
J
= 25°C
C
L
= 1nF, T
J
= 25°C
-
40
-
-
-
13
12
-
-
-
-
0.1
0.4
13.5
13.5
50
50
-
200
0.4
2.1
-
-
300
300
V
uA
V
V
V
V
us
us
V
TH(ST)
V
HYS
-
-
7
0.45
7.7
0.75
8.4
1.05
V
V
V
TH
V
I
I
(LATCH,MIN)
-
-
-
-
250
0
3.0
-
350
-
1.5
1.5
400
V
CC
-
0.8
mV
V
mA
mA
V
IO(C.L)
I
BIAS
V
3
= 0V
V
4
= 0V, Pin 7 open
V
5
= V
REF
, V
6
= 0V
0.45
-
0.5
- 10
0.55
- 30
V
uA
Symbol
Conditions
Min.
Typ. Max. Unit
Maximum Non-Latching Current (Note 5) I
(NONLATCH,MAX)
Notes
:
1. Parameter measured at trip point at latch with V
5
= V
REF
, V
6
= 0V
2. R
T
= 10KΩ, C
T
= 4.7nF
3. Amplifier gain definde as:
∆V7
-
G
= ---------- ;
∆V
4
=
0to1.0V
∆V4
4. Current into Pin 1 guaranteed to latch circuit in shutdown state.
5. Current into Pin 1 guaranteed not to latch circuit in shutdown state.
6. These parameters, although guaranteed over the recommended operating conditions, are not 100% tested in production.
4
UC3846
Td
OUTPUT DEADTIME(T
d
)
12
-
Output deadtime is determined by the external capacitor, C
T
, according to the formula:
Td
(
us
)
=
145C
T
( µF ) 
---------------------------------
3.6
-
12
– --------------------
For large values of R
T
: T
d
(us) = l45C
T
(uF) Oscillator frequency is approximately
R
T
(
KΩ
)
2.2
by the formula:
f
T
(
KHz
)
= -----------------------------------------
-
R
T
(
KΩ
)C
T
( µF )
Figure 1. UC3846 Oscillator Circuit
Figure 2. Error Amplifier Output Configuration
5

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