MIC4684YM データシートの表示（PDF） - Micrel
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MIC4684YM Datasheet PDF : 16 Pages
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Micrel, Inc.
Pin Configuration
Pin Description
Pin Number
1
2, 6, 7
3
4
5
8
SW 1
GND 2
VIN 3
BS 4
8 EN
7 GND
6 GND
5 FB
8-Pin SOP (M)
MIC4684
Pin Name
SW
GND
IN
BS
FB
EN
Pin Function
Switch (Output): Emitter of NPN output switch. Connect to external storage
inductor and Shottky diode.
Ground
Supply (Input): Unregulated +4V to 30V supply voltage (34V transient)
Booststrap Voltage Node (External Component): Connect to external boost
capacitor.
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.
Enable (Input): Logic high = enable; logic low = shutdown
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
VSAT 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, 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
switching regulator’s or controller’s ground pin.Any resistance
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.
Input Voltage (VIN, pin 3)
The VIN 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 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
the AC voltage back to the BS pin plus the dc offset of VIN
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 VSAT of 1.8V, and 75% efficiency
to a VSAT of 0.5V and 88% efficiency respectively.
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.
January 2010
2
M9999-012610
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