Datasheet

MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
12 ______________________________________________________________________________________
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 MAX1742/MAX1842 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 3. 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 50kΩ for R1. R2 is given by the
equation:
where V
REF
is typically 1.1V.
Programming the Switching Frequency
and Off-Time
The MAX1742/MAX1842 features a programmable
PWM mode switching frequency, which is set by the
input and output voltage and the value of R
TOFF
, con-
nected from TOFF to GND. R
TOFF
sets the PMOS
power switch off-time in PWM mode. Use the following
equation to select the off-time according to your
desired switching frequency in PWM mode:
where: t
OFF
= the programmed off-time
V
IN
= the input voltage
V
OUT
= the output voltage
V
PMOS
= the voltage drop across the internal
PMOS power switch
V
NMOS
= the voltage drop across the internal
NMOS synchronous-rectifier switch
f
PWM
= switching frequency in PWM mode
Select R
TOFF
according to the formula:
R
TOFF
= (t
OFF
- 0.07µs) (110kΩ / 1.00µs)
Recommended values for R
TOFF
range from 36kΩ to
430kΩ for off-times of 0.4µs to 4µs.
Inductor Selection
The key inductor parameters must be specified: inductor
value (L) and peak current (I
PEAK
). The following equa-
tion includes a constant, denoted as LIR, which is the
ratio of peak-to-peak inductor AC current (ripple current)
to maximum DC load current. A higher value of LIR allows
smaller inductance but results in higher losses and ripple.
A good compromise between size and losses is found at
approximately a 25% ripple-current to load-current ratio
(LIR = 0.25), which corresponds to a peak inductor cur-
rent 1.125 times the DC load current:
where: I
OUT
= maximum DC load current
LIR = ratio of peak-to-peak AC inductor current
to DC load current, typically 0.25
L
Vt
I LIR
OUT OFF
OUT
=
×
×
t
VV V
fVV V
OFF
IN OUT PMOS
PWM IN PMOS NMOS
–
=
−
()
−+
()
R2 R1
V
V
1
OUT
REF
=−
⎛
⎝
⎜
⎞
⎠
⎟
Figure 4. Adjustable Output Voltage
PIN
2.5V
CC
Output voltage
1.5
1.8REF Output voltage
AdjustableGND
Resistive
divider
FB
OUTPUT
VOLTAGE
(V)
FBSEL
Unconnected Output voltage
Table 3. Output Voltage Programming
LX
R2
R1
R1 = 50kΩ
R2 = R1(V
OUT
/ V
REF
- 1)
V
REF
= 1.1V
FB
V
OUT
MAX1742
MAX1842