Micrel, Inc.

MIC4684

January 2010

2

M9999-012610

Pin Description

Pin Number

Pin Name

Pin Function

1

SW

Switch (Output): Emitter of NPN output switch. Connect to external storage

inductor and Shottky diode.

2, 6, 7

GND

Ground

3

IN

Supply (Input): Unregulated +4V to 30V supply voltage (34V transient)

4

BS

Booststrap Voltage Node (External Component): Connect to external boost

capacitor.

5

FB

Feedback (Input): Outback voltage feedback to regulator. Connect to output

of supply for fixed versions. Connect to 1.23V tap of resistive divider for

adjustable versions.

8

EN

Enable (Input): Logic high = enable; logic low = shutdown

Pin Configuration

1

SW

GND

VIN

BS

8 EN

GND

GND

FB

7

6

5

2

3

4

8-Pin SOP (M)

Detailed Pin Description

Switch (SW, pin 1)

The switch pin is tied to the emitter of the main internal NPN

transistor. This pin is biased up to the input voltage minus the

negative when the output inductor’s magnetic field collapses

at turn-off. During the OFF time the SW pin is clamped by

the output schottky diode to a –0.5V typically.

Ground (GND, pins 2,6,7)

There are two main areas of concern when it comes to the

ground pin, EMI and ground current. In a buck regulator

or any other non-isolated switching regulator the output

capacitor(s) and diode(s) ground is referenced back to the

switchingregulator’sorcontroller’sgroundpin.Anyresistance

between these reference points causes an offset voltage/IR

drop proportional to load current and poor load regulation.

This is why its important to keep the output grounds placed

as close as possible to the switching regulator’s ground pin.

To keep radiated EMI to a minimum its necessary to place

the input capacitor ground lead as close as possible to the

switching regulators ground pin.

This pin is also connected to the internal regulator. The output

diode or clamping diode should have its cathode as close as

possible to this point to avoid voltage spikes adding to the

voltage across the collector.

Bootstrap (BS, pin 4)

The bootstrap pin in conjunction with the external bootstrap

capacitor provides a bias voltage higher than the input volt-

age to the MIC4684’s main NPN pass element.The bootstrap

capacitor sees the dv/dt of the switching action at the SW

pin as an AC voltage. The bootstrap capacitor then couples

where it is rectified and used to provide additional drive to

the main switch, in this case a NPN transistor.

ThisadditionaldrivereducestheNPN’ssaturationvoltageand

Feedback (FB, pin 5)

The feedback pin is tied to the inverting side of a GM error

amplifier. The noninverting side is tied to a 1.235V bandgap

reference. Fixed voltage versions have an internal voltage

divider from the feedback pin. Adjustable versions require an

external resistor voltage divider from the output to ground,

with the center tied to the feedback pin.

Enable (EN, pin 8)

The enable (EN) input is used to turn on the regulator and is

TTL compatible. Note: connect the enable pin to the input if

unused. A logic-high enables the regulator. A logic-low shuts

down the regulator and reduces the stand-by quiescent

input current to typically 150µA. The enable pin has an up-

per threshold of 2.0V minimum and lower threshold of 0.8V

maximum. The hysterisis provided by the upper and lower

thresholds acts as an UVLO and prevents unwanted turn on

of the regulator due to noise.