LTC3836 LINER [Linear Technology], LTC3836 Datasheet - Page 15

• Manufacturer Part Number: LTC3836
• Description: Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The MOSFET power dissipations at maximum output
current are:
Both MOSFETs have I
includes an additional term for transition losses, which are
largest at high input voltages. The bottom MOSFET losses
are greatest at high input voltage or during a short-circuit
when the bottom duty cycle is nearly 100%.
The LTC3836 utilizes a nonoverlapping, antishoot-through
gate drive control scheme to ensure that the MOSFETs
are not turned on at the same time. To function properly,
the control scheme requires that the MOSFETs used are
intended for DC/DC switching applications. Many power
MOSFETs are intended to be used as static switches and
therefore are slow to turn on or off.
P
P
BOT
TOP
=
=
• I
V
V
2.0
1.5
1.0
0.5
V
IN
0
OUT
–50
OUT(MAX)
IN
Figure 4. R
– V
V
IN
•I
OUT
OUT(MAX)
JUNCTION TEMPERATURE (°C)
0
•I
• C
2
OUT(MAX)
R losses and the P
DS(ON)
RSS
2
50
•
• f
vs Temperature
OSC
T
2
• R
•
100
DS(ON)
T
• R
3836 F04
DS(ON)
150
+ 2 • V
TOP
equation
IN
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 LTC3836’s controllers
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 LTC3836 will phase-lock to a clock signal
applied to the SYNC/FCB pin with a frequency between
250kHz and 850kHz (see Phase-Locked Loop and Fre-
quency 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:
I
L
RIPPLE
f
OSC
V
IN
=
– V
•I
V
RIPPLE
V
OUT
IN
OUT
V
•
OUT(MAX)
IN
f
OSC
V
V
OUT
– V
IN
• L
OUT
IN
. Note that the largest ripple
selects 750kHz operation;
OSC
directly determine the
OSC
LTC3836
, is a trade-off
15
3836fa