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MAX1831EEE+T

P1-P3P4-P6P7-P9P10-P12P13-P13
MAX1830/MAX1831
3A, 1MHz, Low-Voltage, Step-Down Regulators with
Synchronous Rectification and Internal Switches
10 ______________________________________________________________________________________
voltage and ground (Figure 4). Regulation is main-
tained for adjustable output voltages when V
FB
= V
REF
.
Use 30kΩ for R1. R2 is given by the equation:
where V
REF
is typically 1.1V.
Programming the Switching
Frequency and Off-Time
The MAX1830/MAX1831 feature a programmable PWM
mode-switching frequency, which is set by the input
and output voltage and the value of R
TOFF
, connected
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: induc-
tor value (L) and peak current (I
PEAK
). The following
equation 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 loss-
es 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 current 1.125 times the DC load cur-
rent:
where:
I
OUT
= maximum DC load current
LIR = ratio of peak-to-peak AC inductor current to DC
load current, typically 0.25
The peak-inductor current at full load is 1.125 x I
OUT
if
the above equation is used; otherwise, the peak current
is calculated by:
Choose an inductor with a saturation current at least as
high as the peak-inductor current. The inductor you
select should exhibit low losses at your chosen operat-
ing frequency.
Capacitor Selection
The input-filter capacitor reduces peak currents and
noise at the voltage source. Use a low-ESR and low-
ESL capacitor located no further than 5mm from IN.
Select the input capacitor according to the RMS input
ripple-current requirements and voltage rating:
where I
RIPPLE
= input RMS current ripple.
The output-filter capacitor affects the output-voltage rip-
ple, output load-transient response, and feedback-loop
stability. For stable operation, the MAX1830/MAX1831
require a minimum output ripple voltage of V
RIPPLE
≥ 1%
✕
V
OUT
.
The minimum ESR of the output capacitor should be:
Stable operation requires the correct output-filter
capacitor. When choosing the output capacitor, ensure
that:
C
t
V
FV s
OUT
OFF
OUT
≥µµ / 79
ESR
L
t
OFF
% >×1
II
VV V
V
RIPPLE LOAD
OUT IN OUT
IN
=
−
()
II
Vt
L
PEAK OUT
OUT OFF
=+
×
×
2
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
=−
MAX1830/MAX1831
3A, 1MHz, Low-Voltage, Step-Down Regulators with
Synchronous Rectification and Internal Switches
______________________________________________________________________________________ 11
Integrator Amplifier
An internal transconductance amplifier fine tunes the
output DC accuracy. A capacitor, C
COMP
, from COMP
to V
CC
compensates the transconductance amplifier.
For stability, choose C
COMP
= 470pF.
A large capacitor value maintains a constant average
output voltage but slows the loop response to changes
in output voltage. A small capacitor value speeds up
the loop response to changes in output voltage but
decreases stability.
High-Current Thermal Considerations
High ambient temperatures can limit the maximum
current or duty factor of the output current, depending
on the total copper, are connected to the MAX1830/
MAX1831 and available airflow.
Figure 5 shows the maximum recommended continuous
output current vs. ambient temperature. Figure 6 shows
the maximum recommended output current vs. the out-
put current duty cycle at high temperatures. These fig-
ures are based on 0.7in
2
of 1oz copper in free air.
Figure 6 assumes that the output current is a square
wave with a 100Hz frequency. The duty cycle is
defined as the duration of the burst current divided by
the period of the square wave. This figure shows the
limitations for continuous bursts of output current.
Note that if the thermal limitations of the MAX1830/
MAX1831 are exceeded, it enters thermal shutdown to
prevent destructive failure.
Frequency Variation with
Output Current
The operating frequency of the MAX1830/MAX1831 is
determined primarily by t
OFF
(set by R
TOFF
), V
IN
, and
V
OUT
as shown in the following formula:
However, as the output current increases, the voltage
drop across the NMOS and PMOS switches increases
and the voltage across the inductor decreases. This
causes the frequency to drop. The change in frequency
can be approximated with the following formula:
∆f
PWM
= -I
OUT
x R
PMOS
/ (V
IN
x t
OFF
)
where R
PMOS
is the resistance of the internal MOSFETs
(50mΩ typ).
Circuit Layout and Grounding
Good layout is necessary to achieve the MAX1830/
MAX1831s’ intended output power level, high efficien-
cy, and low noise. Good layout includes the use of a
ground plane, careful component placement, and cor-
rect routing of traces using appropriate trace widths.
f
VV V
tVV V
PWM
IN OUT PMOS
OFF IN PMOS NMOS
=
−−
()
−+
()
[]
2.40
2.50
2.60
2.70
2.80
3.00
2.90
3.10
3.30
3.20
3.40
3.50
25 4535 55 65 75 85
MAX1830/MAX1831
MAXIMUM RECOMMENDED CONTINUOUS
OUTPUT CURRENT vs. TEMPERATURE
TEMPERATURE (°C)
OUTPUT CURRENT (A)
0.7IN
2
OF 1-OZ COPPER
Figure 5. Maximum Recommended Continuous Output Current
vs. Temperature
2.40
2.60
3.00
2.80
3.20
3.40
04020 60 80 100
MAXIMUM RECOMMENDED BURST CURRENT
vs. BURST CURRENT DUTY CYCLE
DUTY CYCLE (%)
BURST CURRENT (A)
I
OUT
IS A 100Hz SQUARE WAVE
FROM 1A TO THE BURST CURRENT
T
A
= +55°C
T
A
= +85°C
Figure 6. Maximum Recommended Burst Current vs. Burst
Current Duty Cycle
MAX1830/MAX1831
The following points are in order of decreasing impor-
tance:
1) Minimize switched-current and high-current ground
loops. Connect the input capacitor’s ground, the out-
put capacitor’s ground, and PGND. Connect the
resulting island to GND at only one point.
2) Connect the input filter capacitor less than 5mm
away from IN. The connecting copper trace carries
large currents and must be at least 1mm wide,
preferably 2.5mm.
3) Place the LX node components as close together
and as near to the device as possible. This reduces
resistive and switching losses as well as noise.
4) A ground plane is essential for optimum perfor-
mance. In most applications, the circuit is located on
a multilayer board, and full use of the four or more
layers is recommended. Use the top and bottom lay-
ers for interconnections and the inner layers for an
uninterrupted ground plane. Avoid large AC currents
through the ground plane.
3A, 1MHz, Low-Voltage, Step-Down Regulators with
Synchronous Rectification and Internal Switches
12 ______________________________________________________________________________________
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
LX LX
PGND
LX
PGND
V
CC
FBSEL
REF
GND
TOP VIEW
MAX1830
MAX1831
QSOP
IN
LX
COMP
IN
SHDN
TOFF
FB
Pin Configuration
___________________Chip Information
TRANSISTOR COUNT: 3662
P1-P3P4-P6P7-P9P10-P12P13-P13

MAX1831EEE+T

Mfr. #:
Buy MAX1831EEE+T
Manufacturer:
Maxim Integrated
Description:
Switching Voltage Regulators 3A 1MHz Step-Down
Lifecycle:
New from this manufacturer.
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