LTC3736EUF-2#PBF Linear Technology, LTC3736EUF-2#PBF Datasheet - Page 16

• Manufacturer Part Number: LTC3736EUF-2#PBF
• Description: IC CTRLR SW SYNC DUAL 2PH 24QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3736EUF-2#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 9.8 V
• Current - Output: 1A
• Frequency - Switching: 550kHz ~ 750kHz
• Voltage - Input: 2.75 ~ 9.8 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
LTC3736-2
Operating Frequency and Synchronization
The choice of operating frequency, f
between effi ciency and component size. Low frequency
operation improves effi ciency by reducing MOSFET
switching losses, both gate charge loss and transition
loss. However, lower frequency operation requires more
inductance for a given amount of ripple current.
The internal oscillator for each of the LTC3736-2’s control-
lers runs at a nominal 550kHz frequency when the PLLLPF
pin is left fl oating and the SYNC/FCB pin is a DC low or
high. Pulling the PLLLPF to V
pulling the PLLLPF to GND selects 300kHz operation.
Alternatively, the LTC3736-2 will phase-lock to a clock
signal applied to the SYNC/FCB pin with a frequency be-
tween 250kHz and 850kHz (see Phase-Locked Loop and
Frequency Synchronization).
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency f
inductor’s peak-to-peak ripple current:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest effi ciency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specifi ed maximum,
the inductor should be chosen according to:
16
I
L
RIPPLE
≥
f
OSC
V
IN
=
•
–
V
I
RIPPLE
V
V
OUT
IN
OUT
⎛
⎜
⎝
OUT(MAX)
V
•
IN
f
OSC
V
V
–
OUT
IN
V
•
OUT
IN
L
. Note that the largest ripple
selects 750kHz operation;
OSC
⎞
⎟
⎠
directly determine the
OSC
, is a trade-off
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. Core loss is independent of core
size for a fi xed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Schottky Diode Selection (Optional)
The Schottky diodes D1 and D2 in Figure 16 conduct
current during the dead time between the conduction of
the power MOSFETs . This prevents the body diode of the
bottom N-channel MOSFET from turning on and storing
charge during the dead time, which could cost as much as
1% in effi ciency. A 1A Schottky diode is generally a good
size for most LTC3736-2 applications, since it conducts
a relatively small average current. Larger diodes result in
additional transition losses due to their larger junction
capacitance. This diode may be omitted if the effi ciency
loss can be tolerated.
C
The selection of C
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can be
shown that the worst-case capacitor RMS current occurs
when only one controller is operating. The controller with
the highest (V
formula below to determine the maximum RMS capacitor
IN
and C
OUT
Selection
OUT
)(I
IN
is simplifi ed by the 2-phase architec-
OUT
) product needs to be used in the
37362fb