MAX1776EUA 查看數據表（PDF） - Maxim Integrated
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MAX1776EUA Datasheet PDF : 13 Pages
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24V, 600mA Internal Switch, 100% Duty Cycle,
Step-Down Converter
Shutdown (SHDN)
A logic low level on SHDN shuts down the MAX1776
converter. When in shutdown, the supply current drops
to 3µA to maximize battery life, and the internal P-chan-
nel MOSFET turns off to isolate the output from the input.
The output capacitance and load current determine the
rate at which the output voltage decays. A logic level
high on SHDN activates the MAX1776. Do not leave
SHDN floating. If unused, connect SHDN to IN.
Thermal-Overload Protection
Thermal-overload protection limits total power dissipa-
tion in the MAX1776. When the junction temperature
exceeds TJ = +160°C, a thermal sensor turns off the
pass transistor, allowing the IC to cool. The thermal sen-
sor turns the pass transistor on again after the IC’s junc-
tion 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 FB to GND for the 5.0V preset output voltage.
Alternatively, adjust the output voltage by connecting a
voltage-divider from the output to GND (Figure 4).
Select a value for R2 between 10kΩ and 100kΩ.
Calculate R1 with the following equation:
R1
= R2
×
VOUTPUT
VFB
-
1
where VFB = 1.25V, and VOUTPUT may range from
1.25V to VIN.
Setting Current Limit
The MAX1776 has an adjustable peak current limit.
Configure this peak current limit by connecting ILIM
and ILIM2 as shown in Table 1.
INPUT
4.5V TO 24V
CIN
IN
LX
SHDN
ILIM MAX1776 FB
ILIM2
GND
OUT
OUTPUT
L1
1.25V TO VIN
D1
R1
COUT
R2
Figure 4. Adjustable Output Voltage
Table 1. Current-Limit Configuration
CURRENT
LIMIT (mA)
150
300
600
1200
ILIM
CONNECTED TO
GND
GND
IN
IN
ILIM2
CONNECTED TO
GND
IN
GND
IN
Choose a current limit that realistically reflects the maxi-
mum load current. The maximum output current is half
of the peak current limit. Although choosing a lower
current limit allows using an inductor with a lower cur-
rent rating, it requires a higher inductance (see
Inductor Selection) and does little to reduce inductor
package size.
Inductor Selection
When selecting the inductor, consider these four para-
meters: inductance value, saturation rating, series
resistance, and size. The MAX1776 operates with a
wide range of inductance values. For most applica-
tions, values between 10µH and 100µH work best with
the controller’s high switching frequency. Larger induc-
tor values will reduce the switching frequency and
thereby improve efficiency and EMI. The trade-off for
improved efficiency is a higher output ripple and slower
transient response. On the other hand, low-value induc-
tors respond faster to transients, improve output ripple,
offer smaller physical size, and minimize cost. If the
inductor value is too small, the peak inductor current
exceeds the current limit due to current-sense com-
parator propagation delay, potentially exceeding the
inductor’s current rating. Calculate the minimum induc-
tance value as follows:
( ) VIN(MAX) - VOUTPUT × tON(MIN)
L(MIN) =
ILX (PEAK )
where tON(MIN) = 1µs.
The inductor’s saturation current rating must be greater
than the peak switch current limit, plus the overshoot
due to the 250ns current-sense comparator propaga-
tion delay. Saturation occurs when the inductor’s mag-
netic flux density reaches the maximum level the core
can support and the inductance starts to fall. Choose
an inductor with a saturation rating greater than IPEAK
in the following equation:
IPEAK = ILX(PEAK) + (VIN - VOUTPUT) ✕ 250ns / L
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