Design procedure, Table 1. recommended component values (i, Table 2. output voltage programming – Rainbow Electronics MAX1843 User Manual
Page 9
MAX1843
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
_______________________________________________________________________________________
9
Design Procedure
For typical applications, use the recommended compo-
nent values in Table 1. For other applications, take the
following steps:
1) Select the desired PWM-mode switching frequency.
See Figure 3 for maximum operating frequency.
2) Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.
3) Select R
TOFF
as a function of off-time.
4) Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Setting the Output Voltage
The output of the MAX1843 is selectable between one
of three preset output voltages: +2.5V, +1.8V, and
+1.5V. For a preset output voltage, connect FB to the
output voltage, and connect FBSEL as indicated in
Table 2. For an adjustable output voltage, connect
FBSEL to GND, and connect FB to a resistive divider
between the output voltage and ground (Figure 4).
Regulation is maintained for adjustable output voltages
when V
FB
= V
REF
. Use a resistor in the 10k
Ω to 50kΩ
range for R1. R2 is given by the equation:
where V
REF
is typically 1.1V.
R2
R1
V
V
1
OUT
REF
=
−
LX
R2
R1
R2 = R1(V
OUT
/ V
REF
- 1)
V
REF
= 1.1V
FB
V
OUT
MAX1843
Figure 4. Adjustable Output Voltage
0
400
200
800
600
1200
1000
1400
2.6
3.6
4.1
3.1
4.6
5.1
5.6
MAXIMUM RECOMMENDED
OPERATING FREQUENCY vs. INPUT VOLTAGE
MAX1843 fig03
V
IN
(V)
FREQUENCY (kHz)
V
OUT
= +1.5V
V
OUT
= +1.8V
V
OUT
= +2.5V
V
OUT
= +3.3V
Figure 3. Maximum Recommended Operating Frequency vs.
Input Voltage
V
OUT
(V)
R
TOFF
(k
Ω)
2.2
39
L
(µH)
5
3.3
2.2
V
IN
(V)
47
2.2
75
5
1.8
1.5
39
3.3
2.5
1.5
43
3.3
1.8
1.5
56
3.3
1.5
5
2.5
2.2
100
5
1.5
f
PWM
(kHz)
800
850
570
985
940
715
1180
Table 1. Recommended Component
Values (I
OUT
= 2.7A)
Output voltage
Unconnected
FBSEL
OUTPUT
VOLTAGE
(V)
FB
Resistive
divider
GND
Adjustable
Output voltage
REF
1.8
1.5
Output voltage
V
CC
2.5
PIN
Table 2. Output Voltage Programming