LTC3788-1 Datasheet, PDF (16/28 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788-1
APPLICATIONS INFORMATION
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor, due
to the extra switching losses incurred through R1. However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher efficiency at heavy loads.
Peak efficiency is about the same with either method.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. Why would anyone
ever choose to operate at lower frequencies with larger
components? The answer is efficiency. A higher frequency
generally results in lower efficiency because of MOSFET
gate charge and switching losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔI_L decreases with higher
inductance or frequency and increases with higher V_IN:
ΔI_L
=
V_IN
f •L
⎛
⎝⎜ ^1−
V_IN
V_OUT
⎞
⎠⎟
Accepting larger values of ΔI_L allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔI_L = 0.3(I_MAX). The maximum
ΔI_L occurs at V_IN = 1/2 V_OUT.
The inductor value also has secondary effects. The tran-
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
10% of the current limit determined by R_SENSE. Lower
inductor values (higher ΔI_L) will cause this to occur at
lower load currents, which can cause a dip in efficiency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
16
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Actual core loss is independent of core size for a
fixed inductor value, but it is very dependent on inductance
selected. As inductance increases, core losses go down.
Unfortunately, because increased inductance requires more
turns of wire, copper losses will increase.
Ferrite core inductors have very low core loss and are
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard,” which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Power MOSFET Selection
Two external power MOSFETs must be selected for each
controller in the LTC3788-1: one N-channel MOSFET for
the bottom (main) switch, and one N-channel MOSFET
for the top (synchronous) switch.
The peak-to-peak gate drive levels are set by the INTV_CC
voltage. This voltage is typically 5.2V during start-up
(see EXTV_CC pin connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected (V_IN
< 5V); then, sub-logic level threshold MOSFETs (V_GS(TH)
< 3V) should be used. Pay close attention to the BV_DSS
specification for the MOSFETs as well; many of the logic
level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the
on-resistance R_DS(ON), Miller capacitance C_MILLER, input
voltage and maximum output current. Miller capacitance,
C_MILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturer’s data
sheet. C_MILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in V_DS. This result is
then multiplied by the ratio of the application applied V_DS
to the gate charge curve specified V_DS. When the IC is
37881f