MIC4685

Micrel

MIC4685

2

September 2002

Pin Description

Pin Number

Pin Name

Pin Function

1

BS

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

capacitor.

2

IN

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

3

FB

Feedback (Input): Outback voltage feedback to regulator. Connect to 1.235V

tap of resistive divider.

4, Tab

GND

Ground

5

EN

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

6

SW

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

inductor and Schottky diode.

7

NC

No Connect. Tie this pin to ground.

Pin Configuration

7NC

6SW

5EN

4

GND

3FB

2IN

1BS

SPAK-07L (R)

Detailed Pin Description

Switch (SW, Pin 6)

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

V

driven 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, Pin 4, Tab)

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

switching regulator’s or controller’s ground pin. Any resis-

tance 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 it is neces-

sary to place the input capacitor ground lead as close as

possible to the switching regulator’s ground pin.

Input Voltage (V

The V

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 1)

The bootstrap pin in conjunction with the external bootstrap

capacitor provides a bias voltage higher than the input

voltage to the MIC4685’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 the AC voltage back to the BS pin plus the dc offset

of V

to the main switch, in this case a NPN transistor.

This additional drive reduces the NPN’s saturation voltage

and increases efficiency, from a V

efficiency to a V

Feedback (FB, Pin 3)

The feedback pin is tied to the inverting side of an error

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

reference. An external resistor voltage divider is required

from the output to ground, with the center tied to the feedback

pin.

Enable (EN, Pin 5)

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 150

µA. The enable pin has an upper

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.