MIC3808/3809
Micrel
MIC3808/3809
Push-Pull PWM Controller
General Description
The MIC3808 and MIC3809 are a family of complementary
output push-pull PWM control ICs that feature high speed and
low power consumption. The MIC3808/9 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 dual-ended push-pull architecture of the MIC3808/9
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 MIC3808/9 are built on a low-power, high-speed BiCMOS
process. The 130µA start-up current and 1mA run-current
reduce the size of the start-up circuitry and allow high
efficiency even at light loads. The high-speed internal 4MHz
error amplifier allows MIC3808/9 operation up to 1MHz.
The MIC3808 has a turn-on threshold of 12.5V whereas the
MIC3809 has a lower turn-on threshold of 4.3V. Both devices
are available in SOP-8 and MSOP-8 package options 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
Leading edge current-sense blanking
130µA typical start-up current
1mA typical run current
Operation to 1MHz
Internal soft start
On-chip error amplifier with 4MHz gain bandwidth
product
• 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
• Networking power supplies
• Industrial power supplies
• 42V automotive power supplies
• Base stations
Typical Application
V
OUT
12V 100W
V
IN
36V to 75V
Start-Up
Circuitry
MIC3808
RC
VDD
GND OUTA
COMP OUTB
FB
CS
Slope
Comp
Reference
& Isolation
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
MIC3808/3809
MIC3808/3809
Micrel
Ordering Information
Part Number
Standard
MIC3808BM
MIC3809BM
MIC3808BMM
MIC3809BMM
Lead-Free
MIC3808YM
MIC3809YM
MIC3808YMM
MIC3809YMM
Turn On
Threshold
12.5V
4.3V
12.5V
4.3V
Turn Off
Threshold
8.3V
4.1V
8.3V
4.1V
Temperature Range
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
Package
8-Pin SOIC
8-Pin SOIC
8-Pin MSOP
8-Pin MSOP
Pin Configuration
COMP 1
FB 2
CS 3
RC 4
8 VDD
7 OUTA
6 OUTB
5 GND
SOIC-8 (M)
MSOP-8 (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 com-
parator. The error amplifier in the MIC3808 is a true low-output impedance,
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
MIC3808 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. For best stability, keep FB lead
length as short as possible and FB stray capacitance as small as possible.
The input to the PWM, 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.
The oscillator programming pin. The MIC3808’s oscillator tracks V
DD
and
GND internally, so that variations in power supply rails minimally affect
frequency stability. Only two components are required to program the
oscillator, a resistor (tied to the V
DD
and RC), and a capacitor (tied to the RC
and GND). The approximate oscillator frequency is determined by the simple
formula:
2
3
FB
CS
4
RC
F
OSCILLATOR
=
1.41
RC
where frequency is in Hertz, resistance in Ohms, and capacitance in Farads.
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 timing
capacitor lead to GND as short as possible, the timing resistor lead from V
DD
as short as possible, and the leads between timing components and RC as
short as possible. Separate ground and V
DD
traces to the external timing
network are encouraged.
MIC3808/3809
2
April 2005
MIC3808/3809
Micrel
Pin Description
Pin Number
5
6
7
Pin Name
GND
OUTB
OUTA
Pin Function
Ground
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. Although quiescent V
DD
current
is very low, total supply current will be higher, depending on OUTA and
OUTB current, and the programmed oscillator frequency. Total V
DD
current
is the sum of quiescent V
DD
current and the average OUT current. Knowing
the operating frequency and the MOSFET gate charge (Qg), average OUT
current can be calculated from I
OUT
= Qg
×
F, where F is frequency. To
prevent noise problems, bypass V
DD
to GND with a ceramic capacitor as
close to the chip as possible. A 1µF decoupling capacitor is recommended.
8
VDD
April 2005
3
MIC3808/3809
MIC3808/3809
Micrel
Absolute Maximum Ratings
(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, CS) ........................ –0.3V to V
DD
+0.3V
NOT TO EXCEED 6V
Junction Temperature .............................. –55°C to +150°C
Storage Temperature ............................... –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 11
Oscillator Frequency (f
OSC
) ....................... 10kHz to 1MHz
Ambient Temperature (T
A
) ......................... –40°C to +85°C
Package Thermal Resistance
SOIC-8
(θ
JA
) ...................................................... 160°C/W
MSOP-8
(θ
JA
) .................................................... 206°C/W
Electrical Characteristics
T
A
= T
J
= –40°C to +85°C, V
DD
= 10V (Note
9),
1µF capacitor from V
DD
to GND, R = 22KΩ, C = 330pF.
Parameter
Oscillator Section
Oscillator Frequency
Oscillator Amplitude/VDD
Error Amp Section
Input Voltage
Input Bias Current
Open Loop Voltage Gain
COMP Sink Current
COMP Source Current
COMP Clamp Voltage
PWM Section
Maximum Duty Cycle
Minimum Duty Cycle
Current Sense Section
Gain
Maximum Input Signal
CS to Output Delay
CS Source Current
CS Sink Current
Over Current Threshold
COMP to CS 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
CS = 0V
CS = 0.5V, RC = 5.5V,
Note 8
Note 6
(Guaranteed by design)
Note 7
COMP = 3V, CS from 0 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 5
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 4
180
0.44
200
0.5
220
0.56
kHz
V/V
Condition
Min
Typ
Max
Units
MIC3808/3809
4
April 2005
MIC3808/3809
Parameter
Undervoltage Lockout Section
Start Threshold
Minimum Operating Voltage
After Start
Hysteresis
Soft Start Section
COMP Rise Time
Overall Section
Startup Current
Operating Supply Current
VDD Zener Shunt Voltage
Note 1.
Note 2.
Note 3.
Note 4.
Note 5.
Micrel
Condition
Min
Typ
Max
Units
MIC3808,
Note 9
MIC3809
MIC3808
MIC3809
MIC3808
MIC3809
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
FB = 0V, CS = 0V,
Notes 9 and 10
I
DD
= 10mA,
Note 12
13
130
1
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.
Measured at RC. Signal amplitude tracks V
DD
.
The COMP pin is internally clamped to 3.65V(typ). The COMP pin source current is tested at V
COMP
= 3.0V to avoid interfering with this clamp
voltage. The minimum source current increases as V
COMP
approaches V
CLAMP
.
Gain is defined by A =
Note 6.
Note 7.
Note 8.
Note 9.
∆
V
COMP
∆
V
CS
, 0
≤
V
CS
≤
0.4V.
Parameter measured at trip point of latch with FB at 0V.
The internal current sink on the CS pin is designed to discharge an external filter capacitor. It is not intended to be a DC sink path.
For MIC3808, set V
DD
above the start threshold before setting at 10V.
Note 10.
Does not include current in the external oscillator network.
Note 11.
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 to less than the 20mA V
DD
maximum supply current rating.
Note 12.
Start threshold and Zener Shunt threshold track one another.
April 2005
5
MIC3808/3809
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