MAX879C 데이터 시트보기 (PDF) - Maxim Integrated
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MAX879C Datasheet PDF : 12 Pages
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5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
______________________________________________________________Pin Description
PIN
NAME
FUNCTION
1
ILIM
Sets switch current-limit input. Connect to IN for 1A current limit. A resistor from
ILIM to IN sets lower peak inductor currents.
2
IN
Input supply.
3
AGND
Analog ground. Not internally connected to PGND.
4
PGND
Power ground must be low impedance; solder directly to ground plane or star
ground. Connect to AGND, close to the device.
5
LX
1A NPN power switch collector and active-rectifier PNP emitter.
6
OUT
Voltage output. Connect filter capacitor close to pin.
7
–———–
SHDN
Shutdown input disables power supply when low. Also disconnects load from
input. Threshold is set at VIN/2. Connect to IN for normal operation.
N.C.
(MAX877)
No connect, not internally connected.
8
SEL
(MAX878)
Selects the main output voltage: 3.3V when connected to AGND, 3.0V when left
open.
FB
(MAX879)
Feedback input for adjustable-output operation. Connect to an external voltage
divider between VOUT and AGND.
_______________Detailed Description
Operating Principle
The MAX877/MAX878/MAX879 combine a switch-mode
regulator with an NPN bipolar power switch and current
limit, a precision voltage reference, and a synchronous
rectifier—all in a single monolithic device. In shutdown
mode, the internal rectifier is completely turned off and
disconnects the load from the source. Only two external
components are required in addition to the input bypass
capacitor—a 22µH inductor, and a 100µF filter capacitor.
A minimum-off-time, current-limited, pulse-frequency-
modulation (PFM) control scheme combines the high
output power and efficiency of pulse-width modulation
(PWM) with the low quiescent currents of traditional PFM
pulse skippers.
External conditions (inductor value, load, and input volt-
age) determine the way the converter operates, as fol-
lows:
At light loads, the current through the inductor starts at
zero, rises to a peak value, and drops down to zero in
each cycle (discontinuous-conduction mode). In this
case, the switching frequency is governed by a pair of
one-shots, which set a maximum on-time inversely pro-
portional to VIN [tON = 8.8/(VIN - 0.25)] and a minimum
off-time (1.3µs for MAX877/MAX879, or 2.3µs for
MAX878). With a 22µH inductor, LX’s peak current is
about 400mA and is independent of input voltage.
Efficiency at light loads is improved because of lower
peak currents.
At very light loads, more energy is stored in the coil
than is required by the load in each cycle. The converter
regulates by skipping entire cycles. Efficiency is typically
65% to 75% in the pulse-skipping mode. Pulse-skipping
waveforms can be irregular, and the output waveform
contains a low-frequency component. Larger, low equiv-
alent-series-resistance (ESR) filter capacitors can help
reduce the ripple voltage if needed.
At heavy loads above approximately 100mA, the con-
verter enters continuous-conduction mode, where cur-
rent always flows in the inductor. The switch ON state is
controlled on a cycle-by-cycle basis, either by the
tON(max) time or the preset current limit in the switch.
This prevents exceeding the switch current rating or sat-
urating the inductor. At very heavy loads, the inductor
current self-oscillates between this peak current limit and
some lower value governed by the minimum off-time, the
inductance value, and the input/output differential.
6 _______________________________________________________________________________________
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