MIC4043
Micrel
MIC4043
Low-Voltage Secondary-Side Shunt Regulator
General Description
The MIC4043 is a shunt regulator optimized for secondary-
side regulation in low-voltage power supplies. Featuring an
output stage guaranteed to swing within 400mV of ground,
the MIC4043 can be used in power supplies operating down
to 1.8V, even with optoisolators requiring greater than 1.2V of
headroom.
In power supply applications, the MIC4043 normally drives
the LED of an optically isolated feedback circuit. The MIC4043
monitors a resistively-divided output voltage and sinks error
current through the optoisolator’s LED (secondary side); the
optoisolator’s transistor (primary side) provides this signal to
the controller’s feedback input. The MIC4043 is also practical
for other voltage-monitoring applications requiring an open-
collector output.
The MIC4043 replaces conventional ’431-type shunt regula-
tors to allow low-voltage applications where there is inad-
equate headroom for a 2.5V regulator in series with an
optoisolator. Replacing ’431-type devices requires only a
minor change to the way that the resistive-divider values are
calculated.
Features
• Ideal for 1.8V switching converters
• Low-voltage operation
400mV maximum saturation over
operating temperature range
• Easy to use
voltage in, current out
• 2% voltage tolerance over operating temperature range
Applications
• Optically isolated low-volage power supplies
• Low-voltage discrete regulator control
Typical Application
OPTICAL
ISOLATION
COMPENSATION
MIC4043
V
IN
MIC4043
Low-Side Feedback
Control
IN SNK
GND FB
V
OUT
1
7
R1
R2
2
6
MIC38HC43BN
1
2
3
4
COMP
FB
I
SNS
R
T
/C
T
V
REF
V
DD
V
OUT
GND
8
4
7
6
5
Return
3
R2
V
OUT
=
1.245V
+
1
R1
SECONDARY SIDE
PRIMARY SIDE
200kHz DC-DC Flyback Converter
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
May 2000
1
MIC4043
MIC4043
Micrel
Ordering Information
Part Number
MIC4043BM4
Marking
RBAD
Voltage
1.245V
Tolerance
1%
Configuration
Open Collector
Temperature Range
–40°C to +85°C
Package
SOT-143
Pin Configuration
FB
4
GND
3
Part
Identification
RBxx
1
IN
2
SNK
MIC4043
Pin Description
Pin Number
1
2
3
4
Pin Name
IN
SNK
GND
FB
Pin Function
Input: Supply voltage input.
Sink (Output): NPN open collector output.
Ground
Feedback (Input): 1.245V feedback input from external voltage-divider
network.
Absolute Maximum Ratings
(Note 1)
Input Voltage (V
IN
) ...................................................... +15V
Output Voltage (V
SNK
) ................................................ +15V
Storage Temperature (T
S
) ....................... –65°C to +150°C
ESD Rating,
Note 3
human body model .................................................... 2kV
machine model ........................................................ 200V
Operating Ratings
(Note 2)
Input Voltage (V
IN
) ...................................................... +10V
Output Voltage (V
SNK
) ................................................ +10V
Maximum Output Current (I
SNK
) ................................ 15mA
Temperature Range (T
A
) ........................... –40°C to +85°C
MIC4043
2
May 2000
MIC4043
Micrel
Electrical Characteristics
T
A
= 25°C,
bold
values indicate –40°C
≤
T
A
≤
+85°C; unless noted
Parameter
Reference Voltage
Reference Voltage Tolerance
Supply Current
Transconductance
∆I
SINK
/∆V
IN
Output Transistor
Saturation Voltage
Output Leakage
Note 1.
Note 2.
Note 3.
Condition
Min
Typ
1.245
Max
±1
±
2
Units
V
%
%
µA
µA
S
S
I
SNK
= 0mA
1mA < I
SNK
< 15mA
I
SNK
= 15mA
V
SNK
= 5V, output transistor off
3.5
2
35
150
160
65
70
250
400
0.5
1
mV
mV
µA
µA
Exceeding the absolute maximum rating may damage the device.
The device is not guaranteed to function outside its operating rating.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. Machine model, 200pF.
Test Circuits
Floating
Bench
Supply
OUT
RETURN
1k
R
A
MIC4043
IN SNK
GND FB
C
FB
2700pF
R1
Analyzer
R
R2
33k
R
R
Do Not Ground!
* Compensation element
V
OUT
1.8V
2.5V
3.3V
R1
72k
33k
20k
R2
33k
33k
33k
50Ω
OUTPUT
A
A
A
Test Circuit 1. Compensation (Bode Plot) Circuit
Supply
OUT
RETURN
1k
MIC4043
IN SNK
GND FB
R3
2k
R1
C2*
2700pF
R2
33k
C1*
1000pF
* Compensation components
Test Circuit 2. Transient Response Circuit
May 2000
3
MIC4043
MIC4043
Micrel
Transient Response 1a.
1.8V Output Without Compensation
Transient Response 1b.
1.8V Output With Compensation
V
IN
V
REG
V
OUT
V
OUT
V
REG
V
IN
Transient Response 2a.
2.5V Output Without Compensation
Transient Response 2b.
2.5V Output With Compensation
V
IN
V
OUT
V
REG
Transient Response 3a.
3.3V Output Without Compensation
V
OUT
V
REG
V
IN
Transient Response 3b.
3.3V Output With Compensation
V
IN
V
REG
MIC4043
V
OUT
4
V
OUT
V
REG
V
IN
May 2000
MIC4043
Micrel
Functional Diagram
IN
V
REF
1.245V
SNK
FB
MIC4043
GND
Functional Description
The MIC4043 combines a G
m
amplifier, precision 1.245V
reference, and a pass transistor in a single package.
The operation of the MIC4043 is similar to conventional shunt
regulators such as the industry standard ’431. In a closed
loop system, the MIC4043 maintains the desired feedback
voltage at the FB pin by sinking current onto the SNK pin
proportional to the error voltage at the FB pin. The ratio of sink
current to error voltage is the transconductance of the device.
Reference
The MIC4043 uses a
high-side reference
. External voltage
dividers providing feedback to the MIC4043 will be
inverted
when compared to those used with
’
431-equivalent devices.
Behavior
The external feedback voltage is compared to the internal
high-side 1.245V reference.
If the feedback voltage, V
FB
, is less than V
IN
– V
REF
, the
amplifier provides no drive to the sink transistor. If the
feedback voltages is greater than V
IN
– V
REF
, the amplifier
drives the pass transistor which sinks current to ground.
May 2000
5
MIC4043
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