Rainbow Electronics MAX15041 User Manual
Page 11
Inductor Selection
A larger inductor value results in reduced inductor ripple
current, leading to a reduced output ripple voltage.
However, a larger inductor value results in either a larger
physical size or a higher series resistance (DCR) and a
lower saturation current rating. Typically, inductor value
is chosen to have current ripple equal to 30% of load
current. Choose the inductor with the following formula:
where f
SW
is the internally fixed 350kHz switching fre-
quency, and
∆I
L
is the estimated inductor ripple current
(typically set to 0.3 x I
LOAD
). In addition, the peak
inductor current, I
L_PK,
must always be below both the
minimum high-side MOSFET current-limit value,
I
HSCL_MIN
(5A, typ), and the inductor saturation current
rating, I
L_SAT
. Ensure that the following relationship is
satisfied:
Diode Selection
The MAX15041 requires an external bootstrap steering
diode. Connect the diode between V
DD
and BST. The
diode should have a reverse voltage rating, higher than
the converter input voltage and a 200mA minimum cur-
rent rating. Typically, a fast switching or Schottky diode
is used in this application, but a simple low-cost diode
(1N4007) suffices.
Input Capacitor Selection
For a step-down converter, input capacitor C
IN
helps to
keep the DC input voltage steady, in spite of discontin-
uous input AC current. Low-ESR capacitors are pre-
ferred to minimize the voltage ripple due to ESR.
Size C
IN
using the following formula:
Output-Capacitor Selection
Low-ESR capacitors are recommended to minimize the
voltage ripple due to ESR. Total output-voltage peak-to-
peak ripple is estimated by the following formula:
For ceramic capacitors, ESR contribution is negligible:
For tantalum or electrolytic capacitors, ESR contribution
is dominant:
Compensation Design Guidelines
The MAX15041 uses a fixed-frequency, peak-current-
mode control scheme to provide easy compensation
and fast transient response. The inductor peak current is
monitored on a cycle-by-cycle basis and compared to
the COMP voltage (output of the voltage error amplifier).
The regulator’s duty-cycle is modulated based on the
inductor’s peak current value. This cycle-by-cycle con-
trol of the inductor current emulates a controlled current
source. As a result, the inductor’s pole frequency is
shifted beyond the gain-bandwidth of the regulator.
System stability is provided with the addition of a sim-
ple series capacitor-resistor from COMP to SGND. This
pole-zero combination serves to tailor the desired
response of the closed-loop system.
The basic regulator loop consists of a power modulator
(comprising the regulator’s pulse-width modulator,
compensation ramp, control circuitry, MOSFETs, and
inductor), the capacitive output filter and load, an out-
put feedback divider, and a voltage-loop error amplifier
with its associated compensation circuitry. See Figure 1
for a graphical representation.
The average current through the inductor is expressed as:
where I
L
is the average inductor current and G
MOD
is
the power modulator’s transconductance. For a buck
converter:
where R
LOAD
is the equivalent load resistor value.
Combining the two previous equations, the power mod-
ulator’s transfer function in terms of V
OUT
with respect
to V
COMP
is:
V
V
R
I
I
G
R
G
OUT
COMP
LOAD
L
L
MOD
LOAD
MOD
=
Ч
⎛
⎝⎜
⎞
⎠⎟
=
Ч
V
R
I
OUT
LOAD
L
=
×
I
G
V
L
MOD
COMP
=
×
R
f
C
ESR COUT
SW
OUT
_
>>
Ч
Ч
1
8
R
f
C
ESR COUT
SW
OUT
_
<<
Ч
Ч
1
8
∆V
V
f
L
V
V
R
OUT
OUT
SW
OUT
IN
ESR COUT
=
Ч
Ч −
⎛
⎝⎜
⎞
⎠⎟
Ч
+
1
1
_
8
8
Ч
Ч
⎛
⎝⎜
⎞
⎠⎟
f
C
SW
OUT
C
I
f
V
V
V
IN
LOAD
SW
IN RIPPLE
OUT
IN
=
Ч
Ч
∆
_
I
I
I
I
I
L PK
LOAD
L
HSCL MIN L SAT
_
_
_
min(
,
)
=
+ ×
<
1
2
∆
L
V
f
I
V
V
OUT
SW
L
OUT
IN
=
Ч
Ч −
⎛
⎝⎜
⎞
⎠⎟
∆
1
MAX15041
Low-Cost, 3A, 4.5V to 28V Input, 350kHz, PWM
Step-Down DC-DC Regulator with Internal Switches
______________________________________________________________________________________
11