Applications information – Rainbow Electronics MAX8548 User Manual
Page 9
MAX8545/MAX8546/MAX8548
Low-Cost, Wide Input Range, Step-Down
Controllers with Foldback Current Limit
_______________________________________________________________________________________
9
from high-voltage inputs. With high input voltages, the
required duty factor is approximately:
where R
DS(ON)
x I
LOAD
is the voltage drop across the
synchronous rectifier. Therefore, the maximum input
voltage (V
IN(DFMAX)
) that can supply a given output
voltage is:
If the circuit cannot attain the required duty cycle dic-
tated by the input and output voltages, the output volt-
age still remains in regulation. However, there may be
intermittent or continuous half-frequency operation as
the controller attempts to lower the average duty cycle
by deleting pulses. This can increase output voltage
ripple and inductor current ripple, which increases
noise and reduces efficiency. Furthermore, circuit sta-
bility is not guaranteed.
Applications Information
Design Procedures
1) Input Voltage Range. The maximum value
(V
IN(MAX)
) must accommodate the worst-case high
input voltage. The minimum value (V
IN(MIN)
) must
account for the lowest input voltage after drops due
to connectors, fuses, and switches are considered.
In general, lower input voltages provide the best
efficiency.
2) Maximum Load Current. There are two current
values to consider. Peak load current (I
LOAD(MAX)
)
determines the instantaneous component stresses
and filtering requirements and is key in determining
output capacitor requirements. I
LOAD(MAX)
also
determines the required inductor saturation rating.
Continuous load current (I
LOAD
) determines the
thermal stresses, input capacitor, and MOSFETs,
as well as the RMS ratings of other heat-contribut-
ing components such as the inductor.
3) Inductor Value. This choice provides tradeoffs
between size, transient response, and efficiency.
Higher inductance value results in lower inductor
ripple current, lower peak current, lower switching
losses, and, therefore, higher efficiency at the cost
of slower transient response and larger size. Lower
inductance values result in large ripple currents,
smaller size, and poor efficiency, while also provid-
ing faster transient response.
Setting the Output Voltage
An output voltage between 0.8V and (0.83 x V
IN
) can
be configured by connecting FB to a resistive divider
between the output and GND (see Figures 1 and 2).
Select resistor R4 in the 1k
Ω to 10kΩ range. R3 is then
given by:
where V
FB
= +0.8V.
Inductor Selection
Determine an appropriate inductor value with the fol-
lowing equation:
where LIR is the ratio of inductor ripple current to aver-
age continuous maximum load current. Choosing LIR
between 20% to 40% results in a good compromise
between efficiency and economy. Choose a low-core-
loss inductor with the lowest possible DC resistance.
Ferrite-core-type inductors are often the best choice for
performance; however, the MAX8548’s low switching
frequency also allows the use of powdered iron core
inductors in ultra-low-cost applications where efficiency
is not critical. With any core material, the core must be
large enough not to saturate at the peak inductor cur-
rent (I
PEAK
).
Setting the Current Limit
The MAX8545/MAX8546/MAX8548 provide valley cur-
rent limit by sensing the voltage across the external
low-side MOSFET. The minimum current-limit threshold
voltage is -280mV for the MAX8545/MAX8548 and
-140mV for the MAX8546. The MOSFET on-resistance
required to allow a given peak inductor current is:
where I
VALLEY
= I
LOAD(MAX)
x (1 - LIR / 2), and
R
DS(ON)MAX
is the maximum on-resistance of the low-
side MOSFET at the maximum operating junction
temperature.
R
V
I
for the MAX
DS ON MAX
VALLEY
(
)
.
(
)
≤
0 14
8546
R
V
I
for the MAX
MAX
DS ON MAX
VALLEY
(
)
.
(
/
)
≤
0 28
8545
8548
I
I
LIR
I
PEAK
LOAD MAX
LOAD MAX
=
+
×
(
)
(
)
2
L
V
V
V
V
f
LIR I
OUT
IN
OUT
IN
OSC
LOAD MAX
=
Ч
−
(
)
Ч
Ч
Ч
(
)
R
R
V
V
OUT
FB
3
4
1
=
−
V
DC
V
R
I
IN DFMAX
MIN
OUT
DS ON
LOAD
(
)
(
)
≤
+
×
(
)
(
)
1
V
R
I
V
OUT
DS ON
LOAD
IN
+
Ч
(
)
(
)