Datasheet

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