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参数资料
| 型号： | MAX1921EVKIT |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 6/11页 |
| 文件大小： | 0K |
| 描述： | EVAL KIT FOR MAX1921 |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，非隔离 |
| 输出电压： | 1.8V |
| 电流 - 输出： | 400mA |
| 输入电压： | 2 ~ 5.5 V |
| 稳压器拓扑结构： | 降压 |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | MAX1921 |
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
DutyCycle ( MAX ) =
× 100 %
Design Procedure
The MAX1920/MAX1921 are optimized for small external
components and fast transient response. There are
several application circuits (Figures 1 through 4) to
allow the choice between ceramic or tantalum output
capacitor and internally or externally set output volt-
ages. The use of a small ceramic output capacitor is
preferred for higher reliability, improved voltage-posi-
tioning transient response, reduced output ripple, and
the smaller size and greater availability of ceramic versus
tantalum capacitors.
Voltage Positioning
Figures 1 and 2 are the application circuits that utilize
small ceramic output capacitors. For stability, the circuit
obtains feedback from the LX node through R1, while
load transients are fed-forward through C FF . Because
there is no D.C. feedback from the output, the output volt-
age exhibits load regulation that is equal to the output
load current multiplied by the inductor’s series resistance.
This small amount of load regulation is similar to voltage
positioning as used by high-powered microprocessor
supplies intended for personal computers. For the
MAX1920/MAX1921, voltage positioning eliminates or
greatly reduces undershoot and overshoot during load
transients (see the Typical Operating Characteristics ),
which effectively halves the peak-to-peak output voltage
excursions compared to traditional step-down converters.
For convenience, Table 1 lists the recommended external
component values for use with the MAX1921 application
circuit of Figure 1 with various input and output voltages.
In order to calculate the smallest inductor, several cal-
culations are needed. First, calculate the maximum
duty cycle of the application as:
V OUT
V IN ( MIN )
Second, calculate the critical voltage across the inductor as:
if DutyCycle(MAX) < 50%,
then V CRITICAL = (V IN (MIN) - V OUT ),
else V CRITICAL = V OUT
Last, calculate the minimum inductor value as:
L ( MIN ) = 2 . 5 × 10 ? 6 × V CRITICAL
Select the next standard value larger than L(MIN). The
L(MIN) calculation already includes a margin for induc-
tance tolerance. Although values much larger than
L(MIN) work, transient performance, efficiency, and
inductor size suffer.
A 550mA rated inductor is enough to prevent saturation
for output currents up to 400mA. Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support and inductance
falls. Choose a low DC-resistance inductor to improve
efficiency. Tables 2 and 3 list some suggested inductors
and suppliers.
Capacitor Selection
For nearly all applications, the input capacitor, C IN ,
may be as small as 2.2μF ceramic with X5R or X7R
Table 2. Suggested Inductors
Table 1. MAX1921 Suggested
Components for Figure 1
INPUT SOURCE
PART
NUMBER
Coilcraft
LPO1704
L
(μH)
4.7
6.8
10
R L
(ohms max)
0.200
0.320
0.410
Isat (A)
1.10
0.90
0.80
SIZE
6.6 x 5.5 x 1.0
= 36.3mm 3
5V
OUTPUT
3.3V
3.0V
2.5V
3.3V, 1 Li+,
3 x AA
L = 10μH, C OUT = 10μF,
R1 = 8.25k ? , C FF = 3300pF
L = 6.8μH, C OUT = 6.8μF,
R1 = 5.62k ? , C FF = 4700pF
L = 10μH,
2.5V, 2 x AA
N/A
Sumida
CDRH3D16
Sumida
CDRH2D18
Toko
D312F
4.7
6.8
10
4.7
6.8
4.7
10
0.080
0.095
0.160
0.081
0.108
0.38
0.79
0.90
0.73
0.55
0.63
0.57
0.74
0.50
3.8 x 3.8 x 1.8
= 26.0mm 3
3.2 x 3.2 x 2.0
= 20.5mm 3
3.6 x 3.6 x 1.2
= 15.6mm 3
1.8V
1.5V
C OUT = 10μF,
R1 = 8.25k ? ,
C FF = 3300pF
L = 4.7μH, C OUT = 4.7μF,
R1 = 4.75k ? , C FF = 5600pF
Inductor Selection
Toko
D412F
Toko
D52LC
4.7
10
4.7
6.8
0.230
0.490
0.087
0.105
0.84
0.55
1.14
0.95
4.6 x 4.6 x 1.2
= 25.4mm 3
5.0 x 5.0 x 2.0
= 50.0mm 3
10
0.150
0.76
6
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