LTC3836_15 Datasheet, PDF (15/30 Pages) Linear Technology – Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller

LTC3836
APPLICATIONS INFORMATION
The MOSFET power dissipations at maximum output
current are:
P_TOP
=
V_OUT
V_IN
• I_OUT(MAX)2
•
_T
• R_DS(ON)
+2•
V_IN2
• I_OUT(MAX) • C_RSS • f_OSC
P_BOT
=
V_IN
– V_OUT
V_IN
• I_OUT(MAX)2
•
_T
• R_DS(ON)
Both MOSFETs have I²R losses and the P_TOP equation
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%.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3836 F04
Figure 4. R_DS(ON) vs Temperature
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.
Operating Frequency and Synchronization
The choice of operating frequency, f_OSC, is a trade-off
between efficiency and component size. Low frequency
operation improves efficiency 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 floating and the SYNC/FCB pin is a DC low or
high. Pulling the PLLLPF to V_IN selects 750kHz operation;
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_OSC directly determine the
inductor’s peak-to-peak ripple current:
I_RIPPLE
=
V_OUT
V_IN



V_IN – V_OUT
f_OSC • L


Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency 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_OUT(MAX). Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L
^{VIN – VOUT} • ^VOUT
f_OSC •I_RIPPLE V_IN
3836fb
15