MAX5037 MAXIM [Maxim Integrated Products], MAX5037 Datasheet - Page 26

• Manufacturer Part Number: MAX5037
• Description: VRM 9.0, Dual-Phase, Parallelable, Average Current-Mode Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

For example, from the typical VRM specifications in the
Applications Information section with V
the high-side and low-side MOSFET RMS currents are
9.9A and 24.1A, respectively. Ensure that the thermal
impedance of the MOSFET package keeps the junction
temperature at least +25°C below the absolute maxi-
mum rating. Use the following equation to calculate
maximum junction temperature:
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
capacitor discharge) and ∆V
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
converter and N is the number of phases.
For example, at V
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.
VRM 9.0, Dual-Phase, Parallelable,
Average Current-Mode Controller
26
______________________________________________________________________________________
OUT
is the total output current of the multiphase
ESR
C
T
J
IN
= PD
IN
=
OUT
=
I
OUT



N
MOS
∆
I
= 1.75V, the ESR and input
OUT
V
(
N
Q
∆
×
V
×
x θ
D
ESR
ESR
+
f
(
SW
1
J-A
∆
−
2
(caused by the ESR of
)
I
L
D
Input Capacitors
+ T



)
Q
A
(caused by the
OUT
= +1.75V,
(15)
(16)
(17)
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 4 to cal-
culate the peak-to-peak output ripple for a given duty
cycle of two-, four-, and six-phase converters. The maxi-
mum ripple cancellation occurs when N
The allowable deviation of the output voltage during the
fast-transient load dictates 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
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.
Table 4. Peak-to-Peak Output Ripple
Current Calculations
NUMBER OF
PHASES (N)
2
2
4
4
4
6
CYCLE (D)
25 to 50%
RESPONSE
0 to 25%
< 50%
> 50%
> 50%
< 17%
DUTY
) depends on the closed-loop
∆I
∆I
Output Capacitors
∆I
EQUATION FOR I
=
=
=
∆I
∆I
∆I
V
V
(
=
V
O
O
2
=
IN
=
V
(
(
× × ×
D L f
PH
1 2
2
V
O
L f
V
−
D L f
−
D
L f
O
L f
× ×
O
L f
(
×
V
1 2
= K / D.
×
−
(
×
(
1 6
×
O
D
1 4
−
SW
1 3 4
−
SW
SW
−
)(
)(
)
SW
(
D
SW
4
2
D
D
D
D
SW
)
−
)
)
−
−
P-P
D
1
1
)
)
)