MIC3838/3839
Micrel
MIC3838/3839
Flexible Push-Pull PWM Controller
General Description
The MIC3838 and MIC3839 are a family of complementary
output push-pull PWM control ICs that feature high speed and
low power consumption. The MIC3838/9 are ideal for telecom
level (36V to 75V) isolated step down dc/dc conversion
applications where high output current, small size, and high
efficiency are required
The MIC3838/9 are designed for high flexibility with minimum
pin-count. The devices are easily configurable for either
voltage-mode or current-mode control. Additionally, the
MIC3838/9 can easily implement a volt-second clamp that
automatically limits the duty cycle during input transients,
allowing designers to use the smallest possible transformers
and power components. A 3V reference output is also avail-
able that eliminates the need for an external reference.
The dual-ended push-pull architecture of the MIC3838/9
allows more efficient utilization of the transformer than single-
ended topologies, allowing smaller size dc/dc solutions.
Additionally, the out-of-phase push-pull topology allows a
higher effective duty cycle, reducing input and output ripple
as well as stress on the external components. The dead-time
between the two outputs is adjustable between 60ns to
200ns, limiting the duty cycle of each output stage to less than
50%.
The MIC3838 has a turn-on threshold of 12.5V whereas the
MIC3839 has a lower turn-on threshold of 4.3V. Both devices
are available in a small size MSOP-10 package with an
operating range of –40°C to +85°C.
Data sheets and support documentation can be found on
Micrel’s web site at www.micrel.com.
Features
•
•
•
•
•
•
•
•
•
Dual output drive stages in push-pull configuration
Configurable for current-mode or voltage-mode control
Easily implements volt-second clamp
Leading edge current-sense blanking
3V reference output available
130µA typical start-up current
1mA typical run current
Operation to 1MHz
On-chip error amplifier with 4MHz gain bandwidth
product
• Internal soft start
• On-chip V
DD
clamping
• Output drive stages capable of 500mA peak source
current, 1A peak sink current
Applications
• High efficiency “brick” power supply modules
• Half bridge converters
• Full bridge converters
• Push-pull converters
• Voltage-fed push-pull converters
• Telecom equipment and power supplies
• Industrial power supplies
• 42V automotive power supplies
• Base stations
• Networking power supplies
Typical Application
V
IN
36V to 75V
V
OUT
12V 100W
Start-Up
Circuitry
MIC3838x-x
VREF
VDD
GND
OUTB
COMP
FB
RAMP
ILIM
Driver
Feed Forward
Ramp/
Volt Sec Clamp
Reference
& Isolation
RC
OUTA
Voltage-Mode Half-Bridge Converter CIrcuit
Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
April 2005
1
MIC3838/3839
MIC3838/3839
Micrel
Ordering Information
Part Number
Standard
MIC3838BMM
MIC3839BMM
Lead-Free
MIC3838YMM
MIC3839YMM
Turn On
Threshold
12.5V
4.3V
Turn Off
Threshold
8.3V
4.1V
Temperature Range
–40°C to +85°C
–40°C to +85°C
Package
10-Pin MSOP
10-Pin MSOP
Pin Configuration
COMP 1
FB 2
ILIM 3
RAMP 4
RC 5
10 VREF
9 VDD
8 OUTA
7 OUTB
6 GND
MSOP-10 (MM)
Pin Description
Pin Number
1
Pin Name
COMP
Pin Function
COMP is the output of the error amplifier and the input of the PWM
comparator. The error amplifier in the MIC3838 is a true low-output imped-
ance, 4MHz operational amplifier. As such, the COMP pin can both source
and sink current. However, the error amplifier is internally current limited, so
that zero duty cycle can be externally forced by pulling COMP to GND.
The MIC3838 family features built-in full cycle soft start. Soft start is imple-
mented as a clamp on the maximum COMP voltage.
The inverting input to the error amplifier.
The input to the peak current, and overcurrent comparators. The
overcurrent comparator is only intended for fault sensing. Exceeding the
overcurrent threshold will cause a soft start cycle. An internal MOSFET
discharges the current sense filter capacitor to improve dynamic perfor-
mance of the power converter.
Input to the PWM comparator. Sawtooth ramp for PWM control. Allows for
either current-mode or voltage-mode control. An internal MOSFET dis-
charges the current sense filter capacitor.
The oscillator programming pin. Only two components are required to
program the oscillator, a resistor (tied between V
DD
and RC), and a capaci-
tor (tied between RC and GND). The approximate oscillator frequency is
determined by the simple formula:
2
3
FB
ILIM
4
RAMP
5
RC
F
OSCILLATOR
=
1.41
R
×
C
The recommended range of timing resistors is between 7kΩ and 200kΩ and
range of timing capacitors is between 100pF and 1000pF. Timing resistors
less than 7kΩ should be avoided. For best performance, keep the leads
between components as short as possible. Separate ground and VDD
traces to the external timing network are encouraged.
6
GND
Ground. Return path for signal and gate drive functions.
MIC3838/3839
2
April 2005
MIC3838/3839
Micrel
Pin Description
Pin Number
7, 8
Pin Name
OUTB, OUTA
Pin Function
Alternating high current output stages. Both stages are capable of driving
the gate of a power MOSFET. Each stage is capable of 500mA peak source
current, and 1A peak sink current.
The output stages switch at half the oscillator frequency, in a push/pull
configuration. When the voltage on the RC pin is rising, one of the two
outputs is high, but during fall time, both outputs are off. This “dead time”
between the two outputs, along with a slower output rise time than fall time,
insures that the two outputs can not be on at the same time. This dead time
is typically 60ns to 200ns and depends upon the values of the timing
capacitor and resistor.
The high-current output drivers consist of MOSFET output devices, which
switch from V
DD
to GND. Each output stage also provides a very low
impedance to overshoot and undershoot. This means that in many cases,
external Schottky clamp diodes are not required.
The power input connection for this device. Total V
DD
current is the sum of
quiescent V
DD
current and the average gate drive (OUT) current. Knowing
the operating frequency and the MOSFET gate charge (Qg), average OUT
current can be calculated from IOUT = Qg • F, where Qg is the total gate
change of all MOSFETs (OUTA and OUTB) and F is oscillator switching
frequency. To prevent noise problems, bypass VDD to GND with a ceramic
capacitor as close to the chip as possible. A 1µF decoupling capacitor is
recommended.
Internal 3V supply. Will source 1mA maximum.
9
VDD
10
VREF
April 2005
3
MIC3838/3839
MIC3838/3839
Micrel
Absolute Maximum Rating
(Note 1)
Supply Voltage (I
DD
≤10mA)
...................................... +15V
Supply Current ........................................................... 20mA
OUTA/OUTB Source Current (peak) ...................... –0.5A
OUTA/OUTB Sink Current (peak) ............................ 1.0A
COMP Pin .................................................................... V
DD
Analog Inputs (FB, ILIM, RAMP) ......... –0.3V to V
DD
+0.3V
NOT TO EXCEED 6V
Junction Temperature .............................. –55°C to +150°C
Storage Temperature (T
S
) ....................... –65°C to +150°C
Lead Temperature (soldering, 10 sec.) ................... +300°C
ESD Rating,
Note 3
...................................................... 2kV
Operating Ratings
(Note 2)
V
DD
Input Voltage (V
DD
) ..........................................
Note 4
Oscillator Frequency (f
OSC
) ....................... 10kHz to 1MHz
Ambient Temperature (T
A
) ......................... –40°C to +85°C
Package Thermal Resistance
MSOP-10 (θ
JA
) .................................................. 115°C/W
Electrical Characteristics
(Note 5)
T
A
= T
J
= –40°C to +85°C, V
DD
=10V,
Note 10,1µF
capacitor from V
DD
to GND, R=22kΩ, C=330pF.
Parameter
Internal Reference Section
Output voltage
Line Regulation
Load Regulation
Oscillator Section
Oscillator Frequency
Oscillator Amplitude/V
DD
Error Amp Section
Input Voltage
Input Bias Current
Open Loop Voltage Gain
COMP Sink Current
COMP Source Current
COMP PM Clamp Voltage
PWM Section
Maximum Duty Cycle
Minimum Duty Cycle
Current Sense Section
Gain
I
LIM
Maximum Input Signal
I
LIM
to Output Delay
Ramp or I
LIM
Source Current
Ramp or I
LIM
Sink Current
I
LIM
Over Current Threshold
COMP to Ramp Offset
Output Section
OUT Low Level
OUT High Level
Rise Time
Fall Time
I = 100mA
I = –50mA, V
DD
– OUT
C
L
= 1nF
C
L
= 1nF
0.5
0.5
25
25
1
1
60
60
V
V
ns
ns
Ramp = I
LIM
= 0V
Ramp = I
LIM
= 0.5V, RC = 5.5V
Note 10
Guaranteed by design,
Note 8
Note 9
COMP = 3V, I
LIM
from 0mV to 600mV
–200
5
0.7
0.35
10
0.75
0.8
0.8
1.2
1.9
0.45
2.2
0.5
70
2.5
0.55
200
V/V
V
ns
nA
mA
V
V
Measured at OUTA or OUTB
COMP = 0V
48
49
50
0
%
%
(Guaranteed by design)
FB = 2.2V, COMP = 1V
FB = 1.3V, COMP = 3V,
Note 7
V
FB
= 0V
COMP = 2V
1.95
–1
60
0.3
–0.15
3.1
80
2.5
–0.5
3.6
4.0
2
2.05
1
V
µA
dB
mA
mA
V
Note 6
180
0.44
200
0.5
220
0.56
kHz
V/V
DD
I
OUT
= 0mA
MIC3838 9V
≤
V
DD
≤12V
MIC3839 5V
≤
V
DD
≤12V
I
OUT
= 1mA
2.85
3.0
2
14
3.15
10
30
V
mV
mV
Condition
Min
Typ
Max
Units
MIC3838/3839
4
April 2005
MIC3838/3839
Parameter
Undervoltage Lockout Section
Start Threshold
MIC3838,
Note 11
MIC3839
Minimum Operating Voltage
After Start
Hysteresis
MIC3838
MIC3839
MIC3838
MIC3839
Soft Start Section
COMP Rise Time
Overall Section
Startup Current
Operating Supply Current
V
DD
Zener Shunt Voltage
Note 1.
Note 2.
Note 3.
Note 4.
Note 5.
Note 6.
Note 7.
Micrel
Condition
Min
Typ
Max
Units
11.5
4.1
7.6
3.9
3.5
0.1
12.5
4.3
8.3
4.1
4.2
0.2
13.5
4.5
9
4.3
5.1
0.3
V
V
V
V
V
V
FB = 1.8V, Rise from 0.5V to 3V
2.5
20
ms
V
DD
< Start Threshold (MIC3839)
FB = 0V, Ramp = I
LIM
= 0V,
Notes 11, 12
I
DD
= 10mA,
Note 13
13
130
1.5
14
260
2
15
µA
mA
V
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.
Maximum operating voltage is equal to the V
DD
(zener) shunt voltage. When operating at or near the shunt voltage, care must be taken to limit
the V
DD
pin current less than the 20mA V
DD
maximum current rating.
Specification for packaged product only.
Measured at RC.
COMP pin is internally clamped to 3.65V(typ.). COMP pin source current is measured at V
COMP
= 3.0V to avoid interferring with clamp.
Minimum source current is higher as V
COMP
approaches V
CLAMP
.
Gain is defined by A =
Note 8.
Note 9.
∆
V
COMP
∆
V
CS
, 0
≤
V
CS
≤
0.4V.
Parameter measured at trip point of latch with FB at 0V
Note 10.
The internal current sink on the Ramp and I
LIM
pin are designed to discharge an external filter capacitor. It is not intended to be a DC sink
path. Internal discharge FET should be able to discharge the Volt-Sec clamp and feed-forward circuits in the figure below within 50ns.
Note 11.
For MIC3838, set V
DD
above the start threshold before setting at 10V.
Note 12.
Does not include current in the external oscillator network.
Note 13.
Start threshold and Zener Shunt threshold track one another.
April 2005
5
MIC3838/3839
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