LTC3809-1_15 Datasheet, PDF (14/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking

LTC3809-1
APPLICATIONS INFORMATION
that do not work well at high input voltages (e.g., 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.
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 LTC3809-1 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I_BURST(PEAK)
=
1
4
•
ΔV_SENSE(MAX)
R_DS(ON)
The corresponding average current depends on the
amount of ripple current. Lower inductor values (higher
I_RIPPLE) will reduce the load current at which Burst Mode
operation begins.
The ripple current is normally set so that the inductor current
is continuous during the burst periods. Therefore,
I_RIPPLE ≤ I_BURST(PEAK)
This implies a minimum inductance of:
L_MIN
≤
V_IN – V_OUT
f_OSC •I_BURST(PEAK)
•
V_OUT
V_IN
A smaller value than L_MIN could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
efficiency. In general, though, it is a good idea to keep
I_RIPPLE comparable to I_BURST(PEAK).
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. Actual core loss is independent of core size for a
fixed inductor value, but is very dependent on the induc-
tance 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 losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design current
is exceeded. Core saturation results in an abrupt increase
in inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
38091fc
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