MIC4684(2001) 데이터 시트보기 (PDF) - Micrel
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MIC4684 Datasheet PDF : 16 Pages
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MIC4684
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)
Micrel
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 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 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
voltage 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 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.
MIC4684
2
July 2001
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