LTC3737 Datasheet, PDF (13/24 Pages) Linear Technology – Dual 2-Phase, No RSENSE DC/DC Controller with Output Tracking

LTC3737
APPLICATIO S I FOR ATIO
about 10°C in most applications. For a maximum ambient
temperature of 70°C, using ρ_80°C ~ 1.3 in the above
equation is a reasonable choice.
The power dissipated in the MOSFET strongly depends on
its respective duty cycles and load current. When the
LTC3737 is operating in continuous mode, the duty cycles
for the MOSFET are:
Duty Cycle = ^{VOUT + VD}
V_IN + V_D
The MOSFET power dissipations at maximum output
current are:
( ) ^PP
=
V_OUT + V_D
V_IN + V_D
•
I_OUT(MAX)
2
• ρ_T
• R_DS(ON) + k •
V_IN2 • I_OUT(MAX) • ρ_T • R_DS(ON)
The MOSFET has I²R losses and the P_P equation includes
an additional term for transition losses, which are largest
at high input voltages. The constant k = 2A^–1 can be used
to estimate the amount of transition loss.
Using a Sense Resistor
A sense resistor R_SENSE can be connected between SENSE⁺
and SW to sense the output load current. In this case, the
source of the P-channel MOSFET is connected to the SW
pin and the drain is not connected to any pin of the
LTC3737. Therefore, the current comparator monitors the
voltage developed across R_SENSE instead of V_DS of the
P-channel MOSFET. The output current that the LTC3737
can provide in this case is given by:
I_OUT(MAX)
=
∆V_SENSE(MAX)
R_SENSE
–
I_RIPPLE
2
Setting ripple current as 40% of I_OUT(MAX) and using
Figure 2 to choose SF, the value of R_SENSE is:
R_SENSE
=
5
6
•
SF
•
∆V_SENSE(MAX)
I_OUT(MAX)
(See the R_DS(ON) selection in Power MOSFET Selection).
Variation in the resistance of a sense resistor is much
smaller than the variation in on-resistance of the external
MOSFET. Therefore the load current is well controlled, and
the system is more stable with a sense resistor. However
the sense resistor causes extra I²R losses in addition to the
I²R losses of the MOSFET. Therefore, using a sense
resistor lowers the efficiency of LTC3737, especially for
large load current.
Operating Frequency and Synchronization
The choice of operating frequency, f_OSC, is a tradeoff
between efficiency and component size. Low frequency
operation improves efficiency by reducing MOSFET switch-
ing losses, both gate charge loss and transition loss.
However, lower frequency operation requires more induc-
tance for a given amount of ripple current.
The internal oscillator for each of the LTC3737’s control-
lers runs at a nominal 550kHz frequency when the PLLLPF
pin is left floating and the SYNC/MODE 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 LTC3737 will phase lock to a clock signal
applied to the SYNC/MODE 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_IN
–
V_OUT
)^{}
(^V_OUT
+
V_D) / ₍V_IN
f_OSC • L
+
V_{D )} 

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
3737f
13