Rainbow Electronics MAX1843 - Manual (Page 10)

MAX1843
2.7A, 1MHz, Low-Voltage, Step-Down Regulator with
Internal Synchronous Rectification in QFN Package
10 ______________________________________________________________________________________
Programming the Switching Frequency
and Off-Time
The MAX1843 features a programmable PWM mode
switching frequency, which is set by the input and out-
put 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 the desired switching frequen-
cy 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) (110k Ω / 1.00µs)
Recommended values for R TOFF range from 36k Ω to
430k
Ω 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 operating
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 MAX1843 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 capac-
itor. 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.
C
t
V
FV s
OUT
OFF
OUT
≥ µ µ / 79
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
L
V t
I LIR
OUT OFF
OUT
=
Ч
Ч
t
V V V
f V V V
OFF
IN OUT PMOS
PWM IN PMOS NMOS
=
( )
+
( )
− −
−