LTC3808 Datasheet, PDF (15/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
APPLICATIO S I FOR ATIO
V_IN > 5V) may work fine at lower voltages (e.g., 3.3V).
Selecting the N-channel MOSFET is typically easier, since
for a given R_DS(ON), the gate charge and turn-on and turn-
off delays are much smaller than for a P-channel MOSFET.
Using a Sense Resistor
A sense resistor R_SENSE can be connected between SENSE⁺
and SENSE^– to sense the output load current. In this case,
the source of the P-channel MOSFET is connected to
SENSE^– pin and the drain is connected to SW pin of
LTC3808. 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 LTC3808
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 1 to choose SF, the value of R_SENSE is:
R_SENSE
=
5
6
•
SF
•
∆V_SENSE(MAX)
I_OUT(MAX)
See the P-channel 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 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
LTC3808, especially for large load current.
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 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 the LTC3808’s controller runs at
a nominal 550kHz frequency when the PLLLPF pin is left
floating and the SYNC/MODE pin is not configured for
spread spectrum operation. Pulling the PLLLPF to V_IN
selects 750kHz operation; pulling the PLLLPF to GND
selects 300kHz operation.
Alternatively, the LTC3808 will phase-lock to a clock signal
applied to the SYNC/MODE pin with a frequency between
250kHz and 750kHz (see Phase-Locked Loop and Fre-
quency Synchronization).
To further reduce EMI, the nominal 550kHz frequency will
be spread over a range with frequencies between 460kHz
and 635kHz when spread spectrum modulation is
enabled (see Spread Spectrum Modulation with
SYNC/MODE and PLLLPF Pins).
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
Burst Mode Operation Considerations
The choice of R_DS(ON) and inductor value also determines
the load current at which the LTC3808 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I_BURST(PEAK)
=
1
4
•
∆V_SENSE(MAX)
R_DS(ON)
3808f
15