©2000 Fairchild Semiconductor International
www.fairchildsemi.com
Rev. 5.0
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
UC3846
SMPS Controller
UC3846
2
Absolute Maximum Ratings
Electrical Characteristics
(V
CC
=15V, T
A
=0
°
C to +70
°
C, unless otherwise specified)
Parameter
Symbol
Value
Unit
Supply Voltage
V
CC
40
V
Collector Supply Voltage
V
C
40
V
Output Current, Sink of Source (Peak)
I
O
500
mA
Reference Output Current
I
REF
30
mA
Soft Start Sink Current
I
SINK(S.S)
50
mA
Sync Output Current
I
SYNC
5
mA
Error Amplifier Output Current
I
O(E.A)
5
mA
Oscillator Changing Current
I
CHG(OSC)
5
mA
Power Dissipation (T
A
= 25
°
C)
P
D
1000
mW
Operating Temperature
T
OPR
0 ~ +70
°
C
Storage Temperature
T
STG
-65 ~ +150
°
C
Lead Temperature (Soldering, 10sec)
T
LEAD
+300
°
C
Parameter
Symbol
Conditions
Min.
Typ.
Max.
Unit
REFERENCE SECTION
Reference Output Voltage
V
REF
T
J
= 25
°
C, I
REF
= 1mA
5.00
5.10
5.20
V
Line Regulation
V
REF
V
CC
= 8 to 40V
-
5
20
mV
Load Regulation
V
REF
I
REF
1 to 10mA
-
3
15
mV
Temperature Stability(Note 6)
ST
T
-
-
0.4
1.0
mV/
°
C
Output Voltage Range (Note 6)
V
REF
Line,Load,Temp
4.95
-
5.25
V
Short Circuit Output Current
I
SC
V
REF
= 0V
-10
-45
-
mA
Output Noise Voltage(Note 6)
V
NO
f = 10Hz to 10KHz, T
J
= 25
°
C
-
100
-
uV
Long-Term Stability(Note 6)
S
T
T
J
= 125
°
C, 1KHz
2
5
8
mV
UC3846
3
Electrical Characteristics
(V
CC
= 15V,T
A
=0
°
C to +70
°
C, unless otherwise specified)
Parameter
Symbol
Conditions
Min.
Typ.
Max.
Unit
OSCILLATOR SECTION (Note 2)
Initial Accuracy
ACCUR
T
J
= 25
°
C
39
43
47
KHz
Frequency Change with Voltage
f/
V
CC
V
CC
= 8 to 40V
-
1
2
%
Frequency Change with
Temperature (Note 6)
f/
T
-
-
1
-
%
Sync Output High Level
V
OH(SYNC)
-
3.9
4.35
-
V
Sync Output Low Level
V
OL(SYNC)
-
-
2.3
2.5
V
Sync Input High Level
V
IH(SYNC)
V
8
= 0V
3.9
-
-
V
Sync Input Low Level
V
IL(SYNC)
V
8
= 0V
-
-
2.5
V
Sync Input Current
I
I(SYNC)
Sync Voltage = 3.9V, V
8
= 0V
-
1.3
1.5
mA
ERROR AMPLIFIER SECTION
Input Offset Voltage
V
IO
-
-
0.5
5
mV
Input Bias Current
I
BIAS
-
-
-0.6
-1
uA
Input Offset Current
I
IO
-
-
40
250
uA
Common-Mode Range
V
CM
V
CC
= 8 to 40V
0
-
V
CC
2
V
Open Loop Voltage Gain
G
VO
V
O
= 1.2 to 3V, V
CM
= 2V
80
105
-
dB
Unity Gain Bandwidth(Note 6)
BW
T
J
= 25
°
C
0.7
1.0
-
MHz
Common Mode Rejection Ratio
CMRR
V
CM
= 0 to 38V, V
CC
= 40V
75
100
-
dB
Power Supply Rejection Ratio
PSRR
V
CC
= 8 to 40V
80
105
-
dB
Output Sink Current
I
SINK
V
IO
= -15mV to 5V, V
7
= 2.5V
2
6
-
mA
Output Source Current
I
SOURCE
R
L
= 15K
-0.4
-0.5
-
mA
High Output Voltage
V
OH
R
L
= 15K
4.3
4.6
-
V
Low Output Voltage
V
OL
-
-
0.7
1
V
CURRENT SENSE AMPLIFIER SECTION
Amplifier Gain (Note 1, 3)
G
V
V
3
= 0V, Pin 1 open
2.5
2.75
3.0
V
Maximum Differential Input
Signal (V
4
- V
3
) (Note 1)
V
I(DIFF,MAX)
R
L
= 15K
, Pin 1 open
1.1
1.2
-
V
Input Offset Voltage (Note 1)
V
IO
V
1
= 0.5V, Pin 1 open
-
5
25
mV
Common Mode Rejection Ratio
CMRR
V
CM
= 1 to 12V
60
83
-
dB
Power Supply Rejection Ratio
PSRR
V
CC
= 8 to 40V
60
84
-
dB
Input Bias Current (Note 1)
I
BIAS
V
1
= 0.5V, Pin 7 open
-
-2.5
-10
uA
Input Offset Current (Note 1)
I
IO
V
1
= 0.5V, Pin 7 open
-
0.08
1
uA
Delay to Outputs (Note 6)
t
D
T
J
= 25
°
C
-
200
500
ns
UC3846
4
Electrical Characteristics
(V
CC
=15V, T
A
=0
°
C to + 70
°
C, unless otherwise specified)
Notes :
1. Parameter measured at trip point at latch with V
5
= V
REF
, V
6
= 0V
2. R
T
= 10K
, C
T
= 4.7nF
3. Amplifier gain definde as:
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.
Parameter
Symbol
Conditions
Min. Typ. Max. Unit
CURRENT LIMIT ADJUST SECTION
Current Limit Offset Voltage (Note 1)
V
IO(C.L)
V
3
= 0V
V
4
= 0V, Pin 7 open
0.45
0.5
0.55
V
Input Bias Current
I
BIAS
V
5
= V
REF
, V
6
= 0V
-
- 10
- 30
uA
SHUTDOWN TERMINAL SECTION
Threshold Voltage
V
TH
-
250
350
400
mV
Input Voltage Range
V
I
-
0
-
V
CC
V
Minimum Latching Current (Note 4)
I
(LATCH,MIN)
-
3.0
1.5
-
mA
Maximum Non-Latching Current (Note 5) I
(NONLATCH,MAX)
-
-
1.5
0.8
mA
UNDER-VOLTAGE LOCKOUT SECTION
Start Threshold
V
TH(ST)
-
7
7.7
8.4
V
Threshold Hysteresis
V
HYS
-
0.45 0.75 1.05
V
OUTPUT SECTION
Collector-Emitter Voltage
V
CEO
-
40
-
-
V
Collector Leakage Current
I
LEAK
V
C
= 40V
-
-
200
uA
Low Output Voltage 1
V
OL
1
I
SINK
= 20mA
-
0.1
0.4
V
Low Output Voltage 2
V
OL
2
I
SINK
= 100mA
-
0.4
2.1
V
High Output Voltage 1
V
OH
1
I
SOURCE
= 20mA
13
13.5
-
V
High Output Voltage 2
V
OH
2
I
SOURCE
= 100mA
12
13.5
-
V
Rise Time (Note 6)
t
R
C
L
= 1nF, T
J
= 25
°
C
-
50
300
us
Fall Time (Note 6)
t
F
C
L
= 1nF, T
J
= 25
°
C
-
50
300
us
TOTAL STANDBY CURRENT
Supply Current
I
CC
-
-
17
21
mA
G
V7
V4
-----------
V
4
0to1.0V
=
;
=
UC3846
5
OUTPUT DEADTIME(T
d
)
Figure 1. UC3846 Oscillator Circuit
Output deadtime is determined by the external capacitor, C
T
, according to the formula:
For large values of R
T
: T
d
(us) = l45C
T
(uF) Oscillator frequency is approximately
by the formula:
Figure 2. Error Amplifier Output Configuration
Td
Td us
( )
145C
T
µ
F
(
)
12
12
3.6
R
T
K
(
)
---------------------
----------------------------------
=
f
T
KHz
(
)
2.2
R
T
K
(
)
C
T
µ
F
(
)
------------------------------------------
=
UC3846
6
Figure 3. Parallel Operation
Figure 4. Pulse By Pulse Current Limiting
Figure 5. Current Sense Amp Connections
A small PC filter may be required in some applications to reduce switch transients
Differential input allows remote, noise free sensing.
UC3846
7
Mechanical Dimensions
Package
#1
#8
#9
#16
6.40
±
0.20
7.62
0.300
2.54
0.100
0.252
±
0.008
0~15
°
0.25
+0.10
0.05
0.010
+0.004
0.002
3.30
±
0.30
0.130
±
0.012
3.25
±
0.20
0.128
±
0.008
19.40
±
0.20
0.764
±
0.008
19.80
0.780
MAX
5.08
0.200
0.38
0.014
MAX
MIN
0.81
0.032
()
0.46
±
0.10
0.018
±
0.004
0.059
±
0.004
1.50
±
0.10
16-DIP
UC3846
8
Ordering Information
Product Number
Package
Operating Temperature
UC3846N
16 DIP
0 ~ + 70
°
C
UC3846
9
UC3846
7/12/00 0.0m 001
Stock#DSxxxxxxxx
2000 Fairchild Semiconductor International
LIFE SUPPORT POLICY
FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES
OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR
INTERNATIONAL. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body,
or (b) support or sustain life, and (c) whose failure to
perform when properly used in accordance with
instructions for use provided in the labeling, can be
reasonably expected to result in a significant injury of the
user.
2. A critical component in any component of a life support
device or system whose failure to perform can be
reasonably expected to cause the failure of the life support
device or system, or to affect its safety or effectiveness.
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