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MAX15053 Datasheet(PDF) 15 Page - Maxim Integrated Products

Part # MAX15053
Description  High-Efficiency, 2A, Current-Mode Synchronous, Step-Down Switching Regulator
PDF  21 Pages
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Manufacturer  MAXIM [Maxim Integrated Products]
Direct Link  https://www.maximintegrated.com/en.html
Logo MAXIM - Maxim Integrated Products

MAX15053 Datasheet(HTML) 15 Page - Maxim Integrated Products

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tOFF1 is the time needed for inductor current to reach the
zero-current crossing limit (~ 0A):
SKIP LIMIT
OFF1
OUT
LI
t
V
−
×
=
During tON and tOFF1, the output capacitor stores a
chargeequalto(seeFigure2):
(
)2
SKIP LIMIT
LOAD
IN
OUT
OUT
OUT
1
1
Lx I
I
x
VV
V
Q
2
−

−+

−

∆
=
During tOFF2(= n x tCK, number of clock cycles skipped),
output capacitor loses this charge:
(
)
OUT
OFF2
LOAD
2
SKIP LIMIT
LOAD
IN
OUT
OUT
OFF2
LOAD
Q
t
I
11
Lx I
I
x
VV
V
t
2 xI
−
∆
=
⇒

−+

−

=
Finally,frequencyinskipmodeis:
SKIP
ON
OFF1
OFF2
1
f
tt
t
=
++
Output ripple in skip mode is:
(
)
(
)
(
)
(
)
OUT RIPPLE
COUT RIPPLE
ESR RIPPLE
SKIP LIMIT
LOAD
ON
OUT
ESR,COUT
SKIP LIMIT
LOAD
SKIP LIMIT
OUT RIPPLE
ESR,COUT
OUT
IN
OUT
SKIP LIMIT
LOAD
VV
V
I
I
xt
C
R
xI
I
L x I
VR
C
xV
V
xI
I
−
−−
−
−
−
−
−
=
+
−
=
+
−

=
+

−


−
To limit output ripple in skip mode, size COUT based on
the above formula. All the above calculations are appli-
cable only in skip mode.
Compensation Design Guidelines
The MAX15053 uses a fixed-frequency, peak-current-mode
control scheme to provide easy compensation and fast tran-
sient 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 control of the inductor current
emulates a controlled current source. As a result, the induc-
tor’s pole frequency is shifted beyond the gain bandwidth of
the regulator. System stability is provided with the addition of
a simple series capacitor-resistor from COMP to GND. 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, current sense and slope compensation ramps,
control circuitry, MOSFETs, and inductor), the capacitive
output filter and load, an output feedback divider, and a
voltage-loop error amplifier with its associated compensation
circuitry.SeeFigure 1.
The average current through the inductor is expressed as:
L
MOD
COMP
IG
V
=
×
where IL is the average inductor current and GMOD is the
power modulator’s transconductance.
Forabuckconverter:
OUT
LOAD
L
VR
I
=
×
where RLOAD is the equivalent load resistor value.
Combining the above two relationships, the power modu-
lator’s transfer function in terms of VOUT with respect to
VCOMP
is:
OUT
LOAD
L
LOAD
MOD
COMP
L
MOD
V
R
I
RG
VI
G
×
=
=
×
The peak current-mode controller’s modulator gain is
attenuated by the equivalent divider ratio of the load
resistance and the current-loop gain’s impedance. GMOD
becomes:
( )
(
)
MOD
MC
LOAD
S
SW
1
G
DC
g
R
1
K
1 D
0.5
fL
=
×

+
×
×−
−

×

where RLOAD = VOUT/IOUT(MAX), fSW is the switching
frequency, L is the output inductance, D is the duty cycle
(VOUT/VIN), and KS is a slope compensation factor calcu-
lated from the following equation:
(
)
SLOPE
SLOPE
SW
MC
S
N
IN
OUT
S
V
f
L g
K1
1
S
VV
×
× ×
= += +
−
where:
SLOPE
SLOPE
SLOPE
SW
SW
V
S
Vf
t
=
=
×
(
)
IN
OUT
N
MC
VV
S
L g
−
=
×
MAX15053
High-Efficiency, 2A, Current-Mode Synchronous,
Step-Down Switching Regulator
www.maximintegrated.com
Maxim Integrated │ 15



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