MAX1837(2002) 데이터 시트보기 (PDF) - Maxim Integrated
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MAX1837 Datasheet PDF : 13 Pages
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24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
Input-Output (Dropout) Voltage
A step-down converter’s minimum input-to-output volt-
age differential (dropout voltage) determines the lowest
useable input supply voltage. In battery-powered sys-
tems, this limits the useful end-of-life battery voltage. To
maximize battery life, the MAX1836/MAX1837 operate
with duty cycles up to 100%, which minimizes the input-
to-output voltage differential. When the supply voltage
approaches the output voltage, the P-channel MOSFET
remains on continuously to supply the load.
Dropout voltage is defined as the difference between
the input and output voltages when the input is low
enough for the output to drop out of regulation. For a
step-down converter with 100% duty cycle, the dropout
voltage depends on the MOSFET drain-to-source on-
resistance (RDS(ON)) and inductor series resistance;
therefore, it is proportional to the load current:
( ) VDROPOUT = IOUT × RDS(ON) + RINDUCTOR
Shutdown (SHDN)
A logic-level low voltage on SHDN shuts down the
MAX1836/MAX1837. When shut down, the supply cur-
rent drops to 3µA to maximize battery life, and the inter-
nal P-channel MOSFET turns off to isolate the output
from the input. The output capacitance and load cur-
rent determine the rate at which the output voltage
decays. A logic-level high voltage on SHDN activates
the MAX1836/MAX1837. Do not leave SHDN floating. If
unused, connect SHDN to IN. When setting output volt-
ages above 5.5V, the shutdown feature cannot be
used, so SHDN must be permanently connected to IN.
The SHDN input voltage slew rate must be greater than
10V/ms.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX1836/MAX1837. When the junction tem-
perature exceeds TJ = +160°C, a thermal sensor turns
off the pass transistor, allowing the IC to cool. The ther-
mal sensor turns the pass transistor on again after the
IC’s junction temperature cools by 10°C, resulting in
a pulsed output during continuous thermal-overload
conditions.
Design Information
Output Voltage Selection
The feedback input features dual-mode operation.
Connect the output to OUT and FB to GND for the pre-
set output voltage. The MAX1836/MAX1837 are sup-
plied with factory-set output voltages of 3.3V or 5V. The
two-digit part number suffix identifies the output voltage
(see the Selector Guide). For example, the
MAX1836EUT33 has a preset 3.3V output voltage.
The MAX1836/MAX1837 output voltage may be adjust-
ed by connecting a voltage divider from the output to
FB (Figure 5). When externally adjusting the output volt-
age, connect OUT to GND. Select R2 in the 10kΩ to
100kΩ range. Calculate R1 with the following equation:
R1=
R2
VOUT
VFB
-1
where VFB = 1.25V, and VOUT may range from 1.25V to
VIN. When setting output voltages above 5.5V, the shut-
down feature cannot be used, so SHDN must be per-
manently connected to IN.
Inductor Selection
When selecting the inductor, consider these four para-
meters: inductance value, saturation current rating,
series resistance, and size. The MAX1836/MAX1837
operate with a wide range of inductance values. For
most applications, values between 10µH and 100µH
work best with the controller’s switching frequency.
Calculate the minimum inductance value as follows:
L(MIN)
=
(VIN(MAX)
-
VOUT ) tON(MIN)
ILIM
where tON(MIN) = 1.0µs. Inductor values up to six times
L(MIN) are acceptable. Low-value inductors may be
smaller in physical size and less expensive, but they
result in higher peak-current overshoot due to current-
sense comparator propagation delay (300ns). Peak-
current overshoot reduces efficiency and could exceed
the current ratings of the internal switching MOSFET
and external components.
INPUT
4.5V OR 24V
CIN
IN
LX
SHDN
FB
MAX1836
MAX1837
GND
OUT
OUTPUT
L1
1.25V TO VIN
D1
R1
COUT
R2
NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.
Figure 5. Adjustable Output Voltage
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