Rainbow Electronics MAX1842 - Manual (Page 13)

MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
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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 MAX1742/MAX1842
requires a minimum output ripple voltage of V
RIPPLE
≥
1% x V OUT .
The minimum ESR of the output capacitor should be:
Stable operation requires the correct output filter capaci-
tor. When choosing the output capacitor, ensure that:
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. Choose the capacitor values that
result in optimal performance.
Soft-Start
Soft-start allows a gradual increase of the internal cur-
rent limit to reduce input surge currents at startup and
at exit from shutdown. A timing capacitor, C
SS , placed
from SS to GND sets the rate at which the internal cur-
rent limit is changed. Upon power-up, when the device
comes out of undervoltage lockout (2.6V typ) or after
the SHDN pin is pulled high, a 4µA constant-current
source charges the soft-start capacitor and the voltage
on SS increases. When the voltage on SS is less than
approximately 0.7V, the current limit is set to zero. As
the voltage increases from 0.7V to approximately 1.8V,
the current limit is adjusted from 0 to the current-limit
threshold (see the Electrical Characteristics).The volt-
age across the soft-start capacitor changes with time
according to the equation:
The soft-start current limit varies with the voltage on the
soft-start pin, SS, according to the equation:
where I LIMIT is the current threshold from the Electrical
Characteristics.
SSI
V V
V
I
LIMIT
SS
LIMIT
=
−
×
.
.
0 7
1 1
V
A t
C
SS
SS
=
× 4 µ
C
t
V
FV s for the MAX
OUT
OFF
OUT
≥ µ µ / 79 1842
C
t
V
FV s for the MAX
OUT
OFF
OUT
≥ µ µ / 33 1742
ESR
L
t
OFF
%
> ×
1
I I
V V V
V
RIPPLE LOAD
OUT IN OUT
IN
=
−
( )
I I
V t
L
PEAK OUT
OUT OFF = +
Ч
Ч
2
0.7V
1.8V
I LIMIT
t
SHDN
0
0
0
V SS (V)
I LIMIT (A)
Figure 5. Soft-Start Current Limit over Time