MIC4685
Micrel
Pin Configuration
7
6
5
4
3
2
1
NC
SW
EN
GND
FB
IN
BS
TAB, GND
SPAK-07L (R)
Pin Description
Pin Number
1
2
3
4, Tab
5
6
7
Pin Name
BS
IN
FB
GND
EN
SW
NC
Pin Function
Bootstrap Voltage Node (External Component): Connect to external boost
capacitor.
Supply (Input): Unregulated +4V to 30V supply voltage (34V transient)
Feedback (Input): Outback voltage feedback to regulator. Connect to 1.235V
tap of resistive divider.
Ground
Enable (Input): Logic high = enable; logic low = shutdown
Switch (Output): Emitter of NPN output switch. Connect to external storage
inductor and Schottky diode.
No Connect. Tie this pin to ground.
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
SAT
of the main NPN pass element. The emitter is also
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
IN
, Pin 2)
The V
IN
pin is the collector of the main NPN pass element.
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
IN
where it is rectified and used to provide additional drive
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
SAT
of 1.8V, and 75%
efficiency to a V
SAT
of 0.5V and 88% efficiency respectively.
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 150µA. 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.
MIC4685
2
September 2002
MICREL [ MICREL SEMICONDUCTOR ]
