Datasheet

MAX5060/MAX5061
0.6V to 5.5V Output, Parallelable,
Average-Current-Mode DC-DC Controllers
______________________________________________________________________________________ 25
where D = V
OUT
/V
IN
, I
DC
= (I
OUT
- ∆I
L
/2) and I
PK
=
(I
OUT
+ ∆I
L
/2).
where C
OSS
is the MOSFET drain-to-source capaci-
tance.
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 7.8A
and 18.5A, respectively for 20A. Ensure that the ther-
mal impedance of the MOSFET package keeps the
junction temperature at least +25°C below the absolute
maximum rating. Use the following equation to calcu-
late maximum junction temperature:
T
J
= (PD
MOS
x θ
JA
) + T
A
where θ
JA
and T
A
are the junction-to-ambient thermal
impedance and ambient temperature, respectively.
Input Capacitors
The discontinuous input-current waveform of the buck
converter causes large ripple currents in the input capac-
itor. The switching frequency, peak inductor current, and
the allowable peak-to-peak voltage ripple reflected back
to the source dictate the capacitance requirement.
Increasing switching frequency or paralleling multiple out-
of-phase converters lowers the peak-to-average current
ratio, yielding a lower input capacitance requirement for
the same load current.
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 ripple
using the following equation:
where I
OUT
is the output current of the converter.
For example, at V
OUT
= 1.8V, the ESR and input capac-
itance are calculated for the input peak-to-peak ripple
of 100mV or less yielding an ESR and capacitance
value of 1.25mΩ and 110µF.
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 buck converter design, the output-current waveform
is continuous and this reduces peak-to-peak ripple cur-
rent in the output capacitor equal to the inductor ripple
current. Calculate the capacitance, the ESR of the out-
put capacitor, and the RMS ripple current rating of the
output capacitor based on the following equations.
where ∆V
OESR
and ∆V
OQ
are the output-ripple contri-
butions due to ESR and the discharge of output capaci-
tor, respectively.
In the dynamic load environment, the allowable devia-
tion of output voltage during the fast transient load dic-
tates the output capacitance and ESR. The output
capacitors supply the load step until the controller
responds with a greater duty cycle. The response time
(t
RESPONSE
) depends on the closed-loop bandwidth of
the converter. The resistive drop across the capacitor
ESR and capacitor discharge causes a voltage drop
during a step load. Use a combination of SP polymer
and ceramic capacitors for better transient load and
ripple/noise performance.
ESR
V
I
C
I
Vf
OUT
OESR
L
OUT
L
OQ SW
=
=
××
∆
∆
∆
∆8
ESR
V
I
I
C
IDD
Vf
IN
ESR
OUT
L
IN
OUT
QSW
=
()
+






=
×
()
×
−
∆
∆
∆
2
1
PD Q V f
CVf
RI
IIDCIPKII
D
MOS LO G DD SW
OSS IN SW
DS ON
RMS LO
RMS LO DC PK
−
−
=××
()
+
×××






+×
()
=++×
()
×
()
−
−
.
()
2
3
14
1
3
2
2
22
IIDCIPKI I
D
RMS HI DC PK−
=++×
()
×
22
3