Datasheet

LTC3561A
11
3561af
APPLICATIONS INFORMATION
size requirements and any radiated fi eld/EMI requirements
than on what the LTC3561A requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3561A applications.
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PART NUMBER VALUE
MAX DC
CURRENT DCR HEIGHT
Toko A914BYW-1R2M=P3:
D52LC
1.2μH 2.15A 44mΩ 2mm
A960AW-1R2M=P3:
D518LC
1.2μH 1.8A 46mΩ 1.8mm
DB3015C-1068AS-1R0N 1.0μH 2.1A 43mΩ 1.5mm
DB3018C-1069AS-1R0N 1.0μH 2.1A 45mΩ 1.8mm
DB3020C-1070AS-1R0N 1.0μH 2.1A 47mΩ 2mm
A914BYW-2R2M-D52LC 2.2μH 2.05A 49mΩ 2mm
A915AY-2ROM-D53LC 2.0μH 3.3A 22mΩ 3mm
Coilcraft LPO1704-122ML 1.2μH 2.1A 80mΩ 1mm
D01608C-222 2.2μH 2.3A 70mΩ 3mm
LP01704-222M 2.2μH 2.4A 120mΩ 1mm
Sumida CR32-1R0 1.0μH 2.1A 72mΩ 3mm
CR5D11-1R0 1.0μH 2.2A 40mΩ 1.2mm
CDRH3D14-1R2 1.2μH 2.2A 36mΩ 1.5mm
CDRH4D18C/LD-1R1 1.1μH 2.1A 24mΩ 2mm
CDRH4D28C/LD-1R0 1.0μH 3.0A 17.5mΩ 3mm
CDRH4D28C-1R1 1.1μH 3.8A 22mΩ 3mm
CDRH4D28-1R2 1.2μH 2.56A 23.6mΩ 3mm
CDRH6D12-1R0 1.0μH 2.80A 37.5mΩ 1.5mm
CDRH4D282R2 2.2μH 2.04A 23mΩ 3mm
CDC5D232R2 2.2μH 2.16A 30mΩ 2.5mm
Taiyo
Yuden
NPO3SB1ROM 1.0μH 2.6A 27mΩ 1.8mm
N06DB2R2M 2.2μH 3.2A 29mΩ 3.2mm
N05DB2R2M 2.2μH 2.9A 32mΩ 2.8mm
Murata LQN6C2R2M04 2.2μH 3.2A 24mΩ 5mm
FDK MIPW3226DORGM 0.9μH 1.4A 80mΩ 1mm
Catch Diode Selection
Although unnecessary in most applications, a small im-
provement in effi ciency can be obtained in a few applica-
tions by including the optional diode D1 shown in Figure 4,
which conducts when the synchronous switch is off. In
pulse skip mode, the synchronous switch is turned off at
a low current and the remaining current will be carried by
the optional diode. It is important to adequately specify
the diode peak current and average power dissipation
so as not to exceed the diode ratings. The main problem
with Schottky diodes is that their parasitic capacitance
reduces the effi ciency, usually negating the possible
benefi ts for LTC3561A circuits. Another problem that a
Schottky diode can introduce is higher leakage current at
high temperatures, which could reduce the low current
effi ciency.
Remember to keep lead lengths short and observe proper
grounding (see Board Layout Considerations) to avoid ring-
ing and increased dissipation when using a catch diode.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V
OUT
/V
IN
.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
I
RMS
≈ I
MAX
V
OUT
(V
IN
− V
OUT
)
V
IN
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ΔI
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
≅ I
OUT
/2. This simple worst case is commonly used
to design because even signifi cant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple cur-
rent ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
the size or height requirements of the design. An additional
0.1μF to 1μF ceramic capacitor is also recommended on
V
IN
for high frequency decoupling, when not using an all
ceramic capacitor solution.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfi ed, the capacitance