MIC3808/3809 Micrel
MIC3808/3809
Push-Pull PWM Controller
General Description Features
The MIC3808 and MIC3809 are a family of complementary Dual output drive stages in push-pull configuration
output push-pull PWM control ICs that feature high speed and Leading edge current-sense blanking
low power consumption. The MIC3808/9 are ideal for telecom 130A typical start-up current
level (36V to 75V) isolated step down dc/dc conversion 1mA typical run current
applications where high output current, small size, and high Operation to 1MHz
efficiency are required. Internal soft start
On-chip error amplifier with 4MHz gain bandwidth
The dual-ended push-pull architecture of the MIC3808/9
product
allows more efficient utilization of the transformer than single-
On-chip V clamping
ended topologies, allowing smaller size dc/dc solutions. DD
Output drive stages capable of 500mA peak source
Additionally, the out-of-phase push-pull topology allows a
current, 1A peak sink current
higher effective duty cycle, reducing input and output ripple
as well as stress on the external components. The dead-time
Applications
between the two outputs is adjustable between 60ns to
High efficiency brick power supply modules
200ns, limiting the duty cycle of each output stage to less than
Half bridge converters
50%.
Full bridge converters
The MIC3808/9 are built on a low-power, high-speed BiCMOS
Push-pull converters
process. The 130A start-up current and 1mA run-current
Voltage-fed push-pull converters
reduce the size of the start-up circuitry and allow high
Telecom equipment and power supplies
efficiency even at light loads. The high-speed internal 4MHz
Networking power supplies
error amplifier allows MIC3808/9 operation up to 1MHz.
Industrial power supplies
The MIC3808 has a turn-on threshold of 12.5V whereas the
42V automotive power supplies
MIC3809 has a lower turn-on threshold of 4.3V. Both devices
Base stations
are available in SOP-8 and MSOP-8 package options with an
operating range of 40C to +85C.
Data sheets and support documentation can be found on
Micrels web site at: www.micrel.com.
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 MIC3808/3809 Micrel
Ordering Information
Part Number
Turn On Turn Off
Standard Lead-Free Threshold Threshold Temperature Range Package
MIC3808BM MIC3808YM 12.5V 8.3V 40C to +85C 8-Pin SOIC
MIC3809BM MIC3809YM 4.3V 4.1V 40C to +85C 8-Pin SOIC
MIC3808BMM MIC3808YMM 12.5V 8.3V 40C to +85C 8-Pin MSOP
MIC3809BMM MIC3809YMM 4.3V 4.1V 40C to +85C 8-Pin MSOP
Pin Configuration
COMP 1 8 VDD
FB 2 7 OUTA
CS 3 6 OUTB
RC 4 5 GND
SOIC-8 (M)
MSOP-8 (MM)
Pin Description
Pin Number Pin Name Pin Function
1 COMP 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.
2 FB 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.
3 CS 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.
and
4 RC The oscillator programming pin. The MIC3808s oscillator tracks V
DD
GND internally, so that variations in power supply rails minimally affect
frequency stability. Only two components are required to program the
and RC), and a capacitor (tied to the RC
oscillator, a resistor (tied to the V
DD
and GND). The approximate oscillator frequency is determined by the simple
1.41
formula: F =
OSCILLATOR
RC
where frequency is in Hertz, resistance in Ohms, and capacitance in Farads.
The recommended range of timing resistors is between 7k and 200k and
range of timing capacitors is between 100pF and 1000pF. Timing resistors
less than 7k 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 traces to the external timing
DD
network are encouraged.
MIC3808/3809 2 April 2005