MAX1920 Maxim, MAX1920 Datasheet - Page 6

• Manufacturer Part Number: MAX1920
• Description: PLASTIC ENCAPSULATED DEVICES
• Manufacturer: Maxim

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.
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
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.
Low-Voltage, 400mA Step-Down
DC-DC Converters in SOT23
Table 1. MAX1921 Suggested
Components for Figure 1
6
OUTPUT
_______________________________________________________________________________________
3.3V
3.0V
2.5V
1.8V
1.5V
C
C
R1 = 8.25kΩ,
FF
R1 = 8.25kΩ, C
R1 = 5.62kΩ, C
OUT
L = 10µH,
L = 6.8µH, C
L = 10µH, C
= 3300pF
5V
= 10µF,
INPUT SOURCE
OUT
OUT
Design Procedure
FF
FF
3.3V, 1 Li+,
R1 = 4.75kΩ, C
Voltage Positioning
L = 4.7µH, C
= 3300pF
= 4700pF
= 10µF,
= 6.8µF,
3 x AA
OUT
FF
FF
2.5V, 2 x AA
. Because
= 5600pF
= 4.7µF,
N/A
In order to calculate the smallest inductor, several cal-
culations are needed. First, calculate the maximum
duty cycle of the application as:
Second, calculate the critical voltage across the inductor as:
Last, calculate the minimum inductor value as:
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.
Table 2. Suggested Inductors
CDRH3D16
CDRH2D18
NUMBER
LPO1704
Coilcraft
Sumida
Sumida
D52LC
D312F
D412F
PART
Toko
Toko
Toko
then V
DutyCycle MAX
L MIN
(
if DutyCycle(MAX) < 50%,
(µH)
4.7
6.8
4.7
6.8
4.7
6.8
4.7
4.7
4.7
6.8
CRITICAL
10
10
10
10
10
else V
L
)
=
(
2 5 10
(ohms max)
CRITICAL
.
×
0.200
0.320
0.410
0.080
0.095
0.160
0.081
0.108
0.230
0.490
0.087
0.105
0.150
= (V
0.38
0.79
R
)
=
L
−
IN
V MIN
6
IN
V
(MIN) - V
Inductor Selection
= V
×
OUT
(
V
CRITICAL
Isat (A)
OUT
1.10
0.90
0.80
0.90
0.73
0.55
0.63
0.57
0.74
0.50
0.84
0.55
1.14
0.95
0.76
)
× 100
OUT
6.6 x 5.5 x 1.0
3.8 x 3.8 x 1.8
3.2 x 3.2 x 2.0
3.6 x 3.6 x 1.2
4.6 x 4.6 x 1.2
5.0 x 5.0 x 2.0
%
= 36.3mm
= 26.0mm
= 20.5mm
= 15.6mm
= 25.4mm
= 50.0mm
),
SIZE
3
3
3
3
3
3