Datasheet

MAX5038/MAX5041
Dual-Phase, Parallelable, Average Current-Mode
Controllers
______________________________________________________________________________________ 23
For example, from the typical specifications in the
Applications Information section with V
OUT
= +1.8V, the
high-side and low-side MOSFET RMS currents are 9.9A
and 24.1A, respectively. Ensure that the thermal imped-
ance of the MOSFET package keeps the junction tem-
perature at least 25°C below the absolute maximum
rating. Use the following equation to calculate maxi-
mum junction temperature:
T
J
= PD
MOS
x θ
J-A
+ T
A
Input Capacitors
The discontinuous input-current waveform of the buck
converter causes large ripple currents in the input
capacitor. The switching frequency, peak inductor cur-
rent, and the allowable peak-to-peak voltage ripple
reflected back to the source dictate the capacitance
requirement. Increasing the number of phases increas-
es the effective switching frequency and lowers the
peak-to-average current ratio, yielding lower input
capacitance requirement.
The input ripple is comprised of ∆V
Q
(caused by the
capacitor discharge) and ∆V
ESR
(caused by the ESR of
the capacitor). Use low-ESR ceramic capacitors with
high ripple-current capability at the input. Assume the
contributions from the ESR and capacitor discharge are
equal to 30% and 70%, respectively. Calculate the
input capacitance and ESR required for a specified rip-
ple using the following equation:
where I
OUT
is the total output current of the multiphase
converter and N is the number of phases.
For example, at V
OUT
= +1.8V, the ESR and input
capacitance are calculated for the input peak-to-peak
ripple of 100mV or less yielding an ESR and capaci-
tance value of 1mΩ and 200µF.
Output Capacitors
The worst-case peak-to-peak and capacitor RMS ripple
current, the allowable peak-to-peak output ripple volt-
age, and the maximum deviation of the output voltage
during step loads determine the capacitance and the
ESR requirements for the output capacitors.
In multiphase converter design, the ripple currents from
the individual phases cancel each other and lower the
ripple current. The degree of ripple cancellation
depends on the operating duty cycle and the number of
phases. Choose the right equation from Table 3 to calcu-
late the peak-to-peak output ripple for a given duty
cycle of two-, four-, and six-phase converters. The max-
imum ripple cancellation occurs when N
PH
= K / D.
C
I
N
DD
Vf
IN
OUT
QSW
=
×−
()
×
1
∆
ESR
V
I
N
I
IN
ESR
OUT L
=
()
+
⎛
⎝
⎜
⎞
⎠
⎟
∆
∆
2
IIIII
D
RMS LO
DC PK
DC PK−
=++×
()
×
−
()
22
1
3
PD Q V f
CVf
RI
MOS LO G DD SW
OSS IN SW
DS ON
RMS LO
−
−
=××
()
+
×××
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
+×
2
3
14
2
2
.
()
Table 3. Peak-to-Peak Output Ripple
Current Calculations
NUMBER OF
PHASES (N)
DUTY
CYCLE (D)
EQUATION FOR ∆I
P-P
2 < 50%
2 > 50%
4 0 to 25%
4
25% to 50%
4 > 50%
6 < 17%
∆I
VD
Lf
O
SW
=
−
×
()12
∆I
VVD
Lf
IN O
SW
=
−
()
−
()
×
21
∆I
VD
Lf
O
SW
=
−
×
()14
∆I
VDD
DLf
O
SW
=
−−
×××
()()12 4 1
2
∆I
VD D
DLf
O
SW
=
−−
××
()( )2134
∆I
VD
Lf
O
SW
=
−
×
()16
(18)
(19)
(20)
(21)
(22)