Datasheet

MAX16955
36V, 1MHz Step-Down Controller
with Low Operating Current
18
Maxim Integrated
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor RMS current requirement (I
RMS
) is
defined by the following equation:
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
SUP
= 2V
OUT
), so
I
RMS(MAX)
= I
LOAD(MAX)
/2.
Choose an input capacitor that exhibits less than +10°C
self-heating temperature rise at the RMS input current
for optimal long-term reliability.
The input-voltage ripple comprises ΔV
Q
(caused by the
capacitor discharge) and ΔV
ESR
(caused by the ESR of
the capacitor). Use low-ESR ceramic capacitors with
high-ripple-current capability at the input. Assume the
contribution from the ESR and capacitor discharge is
equal to 50%. Calculate the input capacitance and ESR
required for a specified input voltage ripple using the
following equations:
ESR
V
I
I
IN
ESR
OUT
L
=
Δ
+
Δ
2
II
VV V
V
RMS LOAD MAX
OUT SUP OUT
SUP
=
−
()
()
Figure 4. Current-Sense Configurations
MAX16955
DH
DL
L
NH
NL
LX
CS
OUT
DL
GND
R
SENSE
C
OUT
C
IN
INPUT (V
IN
)
a) OUTPUT SERIES RESISTOR SENSING
MAX16955
DH
DL
L
NH
NL
LX
CS
OUT
DL
GND
R
DCR
R
CSHL
=
( )
R
DCR
R2
R1 + R2
R1 R2
C
EQ
INDUCTOR
C
OUT
C
IN
INPUT (V
IN
)
b) LOSSLESS INDUCTOR SENSING
R
DCR
=
[
+
]
L
C
EQ
1
R1
1
R2