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参数资料
| 型号： | MAX1836EUT50-T |
| 厂商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分类： | 稳压器 |
| 英文描述： | COIL 390UH 210MA CHOKE SMD |
| 中文描述： | 0.45 A SWITCHING REGULATOR, PDSO6 |
| 封装： | MO-178AB, SOT-23, 6 PIN |
| 文件页数： | 10/13页 |
| 文件大小： | 394K |
| 代理商： | MAX1836EUT50-T |
M
24V Internal Switch, 100% Duty Cycle,
Step-Down Converters
10
______________________________________________________________________________________
The inductor
’
s saturation current rating must be greater
than the peak switching current, which is determined
by the switch current limit plus the overshoot due to the
300ns current-sense comparator propagation delay:
where the switch current-limit (I
LIM
) is typically 312mA
(MAX1836) or 625mA (MAX1837). Saturation occurs
when the inductor
’
s magnetic flux density reaches the
maximum level the core can support, and the induc-
tance starts to fall.
Inductor series resistance affects both efficiency and
dropout voltage (see the
Input-Output Voltage
section).
High series resistance limits the maximum current avail-
able at lower input voltages and increases the dropout
voltage. For optimum performance, select an inductor
with the lowest possible DC resistance that fits in the
allotted dimensions. Typically, the inductor
’
s series
resistance should be significantly less than that of the
internal P-channel MOSFET
’
s on-resistance (1.1
typ).
Inductors with a ferrite core, or equivalent, are recom-
mended.
The maximum output current of the MAX1836/MAX1837
current-limited converter is limited by the peak inductor
current. For the typical application, the maximum out-
put current is approximately:
Output Capacitor
Choose the output capacitor to supply the maximum
load current with acceptable voltage ripple. The output
ripple has two components: variations in the charge
stored in the output capacitor with each LX pulse, and
the voltage drop across the capacitor
’
s equivalent
series resistance (ESR) caused by the current into and
out of the capacitor:
The output voltage ripple as a consequence of the ESR
and output capacitance is:
where I
PEAK
is the peak inductor current (see the
Inductor Selection
section). These equations are suit-
able for initial capacitor selection, but final values
should be set by testing a prototype or evaluation cir-
cuit. As a general rule, a smaller amount of charge
delivered in each pulse results in less output ripple.
Since the amount of charge delivered in each oscillator
pulse is determined by the inductor value and input
voltage, the voltage ripple increases with larger induc-
tance but decreases with lower input voltages.
With low-cost aluminum electrolytic capacitors, the
ESR-induced ripple can be larger than that caused by
the current into and out of the capacitor. Consequently,
high-quality low-ESR aluminum-electrolytic, tantalum,
polymer, or ceramic filter capacitors are required to
minimize output ripple. Best results at reasonable cost
are typically achieved with an aluminum-electrolytic
capacitor in the 100μF range, in parallel with a 0.1μF
ceramic capacitor.
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit
’
s switching.
The input capacitor must meet the ripple-current
requirement (I
RMS
) imposed by the switching currents
defined by the following equation:
For most applications, nontantalum chemistries (ceram-
ic, aluminum, polymer, or OS-CON) are preferred due
to their robustness with high inrush currents typical of
systems with low-impedance battery inputs.
Alternatively, two (or more) smaller-value low-ESR
capacitors can be connected in parallel for lower cost.
Choose an input capacitor that exhibits <+10
°
C tem-
perature rise at the RMS input current for optimal circuit
longevity.
Diode Selection
The current in the external diode (D1) changes abruptly
from zero to its peak value each time the LX switch
turns off. To avoid excessive losses, the diode must
have a fast turn-on time and a low forward voltage. Use
a diode with an RMS current rating of 0.5A or greater,
and with a breakdown voltage >V
IN
. Schottky diodes
are preferred. For high-temperature applications,
Schottky diodes may be inadequate due to their high
leakage currents. In such cases, ultra-high-speed sili-
con rectifiers are recommended, although a Schottky
diode with a higher reverse voltage rating can often
provide acceptable performance.
I
I
V
V
V
-V
RMS
LOAD
OUT
IN
OUT
IN
=
(
)
V
ESR
V
- I
2C
V
-V
V
RIPPLE(ESR)
PEAK
I
RIPPLE(C)
PEAK
L I
OUT
OUT OUT
IN
IN
OUT
2
=
=
(
)
V
V
V
RIPPLE
RIPPLE(ESR)
RIPPLE(C)
≈
+
I
I
OUT(MAX)
PEAK
=
1
2
I
(V
-V
)
PEAK
LIM
I
IN
OUT
L
=
+
ns
300
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