MIC3808BMM-TR Datasheet MIC3809YM-TR, MIC3809YM, MIC3808YM | P1-P3

April 2005 1 MIC3808/3809

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

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

Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com

Typical Application

36V to 75V

MIC3808

OUT

12V 100W

GND

VDD

OUTA

COMP OUTB

Start-Up

Circuitry

Slope

Comp

Reference

& Isolation

MIC3808/3809 Micrel

MIC3808/3809 2 April 2005

Pin Configuration

1COMP

8 VDD

OUTA

OUTB

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.

4 RC The oscillator programming pin. The MIC3808’s oscillator tracks V

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

and RC), and a capacitor (tied to the RC

and GND). The approximate oscillator frequency is determined by the simple

formula:

1.41

OSCILLATOR

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

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

network are encouraged.

Ordering Information

Part Number

Turn On Turn Off

Standard Lead-Free Threshold Threshold Temperature Range Package

MIC3808BM MIC3808YM 12.5V 8.3V –40°C to +85°C 8-Pin SOIC

MIC3809BM MIC3809YM 4.3V 4.1V –40°C to +85°C 8-Pin SOIC

MIC3808BMM MIC3808YMM 12.5V 8.3V –40°C to +85°C 8-Pin MSOP

MIC3809BMM MIC3809YMM 4.3V 4.1V –40°C to +85°C 8-Pin MSOP

April 2005 3 MIC3808/3809

MIC3808/3809 Micrel

Pin Description

Pin Number Pin Name Pin Function

5 GND Ground

6 OUTB 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

7 OUTA 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

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.

8 VDD The power input connection for this device. Although quiescent V

current

is very low, total supply current will be higher, depending on OUTA and

OUTB current, and the programmed oscillator frequency. Total V

current

is the sum of quiescent V

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

to GND with a ceramic capacitor as

close to the chip as possible. A 1µF decoupling capacitor is recommended.

P1-P3 P4-P6 P7-P9 P10-P11

MIC3808BMM-TR

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