MAX879 데이터 시트보기 (PDF) - Maxim Integrated
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MAX879 Datasheet PDF : 12 Pages
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5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
Active Rectifier
The internal active rectifier of the MAX877/MAX878/
MAX879 replaces the external Schottky catch diode in
normal boost operation. The rectifier consists of a PNP
pass transistor and a unique control circuit which, in
shutdown mode, entirely disconnects the load from the
source. This is a distinct advantage over standard boost
topologies, since it prevents battery drain in shutdown.
The MAX877/MAX878/MAX879 can withstand a momen-
tary short at the output in normal operation.
The active rectifier also acts as a zero-dropout regulator
if the input exceeds the regulated output.The device still
switches to deliver power to the output, and the differ-
ence between the input and output voltage appears
across the rectifier. Efficiency is similar to that of a linear
regulator if the MAX877/MAX878/MAX879 are used as
step-down converters. The maximum output current
(IOUT (MAX)) with larger input/output differentials is deter-
mined by package power dissipation. IOUT (MAX) can be
approximated by:
( ) PDISS
IOUT (MAX) ≈ —————— x 0.9
(VIN - VOUT)
Shutdown
(S—H—D—N– )
is
a
Shutdown
high-impedance, active-low
input. Connect it to IN for normal operation. Keeping
S—H—D—N– at ground holds the converters in shutdown
mode. Since the active rectifier is turned off in this
mode, the path from input to load is cut, and the output
effectively drops to 0V. The supply current in shutdown
mode ranges from 4µA at VIN = 1V to 50µA at VIN = 5V.
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threshold. When driven from external logic, S—H—D—N– can
be driven to a higher voltage than VIN, (6.2V max).
Current Limit
Connecting ILIM to IN sets an LX current limit of 1A. For
smaller output power levels that do not require the maxi-
mum peak current, reduce the peak inductor current by
connecting a resistor between ILIM and IN. This opti-
mizes overall efficiency and allows very small, low-cost
coils with lower current ratings. See Figure 2 to select
the resistor (see also Inductor Selection section).
Output Voltage Selection
The MAX877’s output voltage is fixed at 5V. The MAX878’s
output voltage can be set to 3V by leaving the SEL pin
open, or to 3.3V by connecting SEL to AGND.
CURRENT-LIMIT RESISTOR
vs. PEAK INDUCTOR CURRENT
1200
1000
800
600
VIN = 2.5V
400
200
0
0 2 4 6 8 10 12 14
RESISTOR VALUE (kΩ)
Figure 2. Current-Limit Resistor vs. Peak Inductor Current
The MAX879’s output voltage is set by two resistors, R1
and R2 (Figure 3), which form a voltage divider
between the output and the FB pin. The output voltage
can be set from 2.5V to 6.0V by the equation:
VOUT = VREF (R1 + R2)
R2
where VREF = 0.2025V.
To simplify the resistor selection:
( ) R1 = R2 VOUT - 1
VREF
Since the input current at FB has a maximum of 40nA,
large values (10kΩ to 50kΩ for R2) can be used without
significant accuracy loss. For 1% error, the current
through R2 should be at least 100 times FB’s bias current.
When large values are used for the feedback resistors
(R1 > 50kΩ), stray output impedance at FB can add a
“lag” to the feedback response, destabilizing the regula-
tor and creating a larger ripple at the output. Lead
lengths and circuit board traces at the FB node should be
kept short. Reduce ripple by adding a “lead” compensa-
tion capacitor (C3, 100pF to 50nF) in parallel with R1.
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