MAX15041 Maxim Integrated Products, MAX15041 Datasheet - Page 11

• Manufacturer Part Number: MAX15041
• Description: Step-Down DC-DC Regulator
• Manufacturer: Maxim Integrated Products

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
quency, and ∆I
(typically set to 0.3 x I
inductor current, I
minimum high-side MOSFET current-limit value,
I
rating, I
satisfied:
The MAX15041 requires an external bootstrap steering
diode. Connect the diode between V
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.
For a step-down converter, input capacitor C
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
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:
HSCL_MIN
Step-Down DC-DC Regulator with Internal Switches
∆V
OUT
I
L PK
_
IN
=
SW
L_SAT
using the following formula:
f
SW
V
=
OUT
C
(5A, typ), and the inductor saturation current
is the internally fixed 350kHz switching fre-
I
Low-Cost, 3A, 4.5V to 28V Input, 350kHz, PWM
LOAD
IN
×
. Ensure that the following relationship is
L
L
=
L
×
=
is the estimated inductor ripple current
f
⎛
⎝ ⎜
SW
1
+
L_PK,
______________________________________________________________________________________
f
−
SW
1
2
Output-Capacitor Selection
V
V
×
OUT
V
×
OUT
IN
Input Capacitor Selection
∆
×
I
∆
LOAD
must always be below both the
V
∆
I
LOAD
L
IN RIPPLE
⎞
⎠ ⎟
I
L
×
<
_
⎛
⎝ ⎜
×
min(
R
ESR COUT
⎛
⎝ ⎜
). In addition, the peak
1
Inductor Selection
−
I
HSCL MIN L SAT
_
V
Diode Selection
OUT
V
×
IN
V
DD
_
OUT
V
+
⎞
⎠ ⎟
IN
8 8 ×
and BST. The
,
I
f
SW
IN
_
1
×
helps to
C
OUT
)
⎞
⎠ ⎟
For ceramic capacitors, ESR contribution is negligible:
For tantalum or electrolytic capacitors, ESR contribution
is dominant:
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
the power modulator’s transconductance. For a buck
converter:
where R
Combining the two previous equations, the power mod-
ulator’s transfer function in terms of V
to V
COMP
L
LOAD
V
is the average inductor current and G
is:
V
COMP
OUT
R
R
Compensation Design Guidelines
ESR COUT
ESR COUT
is the equivalent load resistor value.
=
_
_
I
V
L
R
OUT
⎛
⎝ ⎜
=
LOAD
G
G
MOD
MOD
I
L
=
<<
>>
×
R
⎞
⎠ ⎟
LOAD
8
8
I
×
L
×
×
V
=
COMP
f
f
SW
SW
R
www.DataSheet4U.com
LOAD
×
1
1
×
×
I
L
C
C
OUT
OUT
OUT
×
G
MOD
with respect
MOD
11
is