MAX1842EEE Maxim Integrated Products, MAX1842EEE Datasheet - Page 12

• Manufacturer Part Number: MAX1842EEE
• Description: DC/DC Switching Regulators
• Manufacturer: Maxim Integrated Products

Specifications of MAX1842EEE

• Case: ssop
• Dc: 02+
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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