LTC3769_15 Datasheet, PDF (17/32 Pages) Linear Technology – 60V Low IQ Synchronous Boost Controller

LTC3769
Applications Information
The sense resistor values are:
R1= ^R1||R2;
R_D
R2
=
R1•R_D
1− R_D
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at V_IN = 1/2V_OUT:
P_{LOSS _ R1}
=
(V_OUT − V_IN)• V_IN
R1
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 in-
terrelated 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. Also, at
higher frequency the duty cycle of body diode conduction
is higher, which results in lower efficiency. 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 = 1/2V_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
25% 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. Once the value of L is known, an
inductor with low DCR and low core losses should be
selected.
Power MOSFET Selection
Two external power MOSFETs must be selected for the
LTC3769: one N-channel MOSFET for the bottom (main)
switch, and one N-channel MOSFET for the top (synchro-
nous) switch.
The peak-to-peak gate drive levels are set by the INTV_CC
voltage. This voltage is typically 5.4V during start-up
(see EXTV_CC pin connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
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 VDS. This result
is then multiplied by the ratio of the application applied
VDS to the gate charge curve specified VDS. When the IC
is operating in continuous mode, the duty cycles for the
top and bottom MOSFETs are given by:
Main Switch Duty Cycle = ^{VOUT − VIN}
V_OUT
Synchronous Switch Duty Cycle = ^VIN
V_OUT
For more information www.linear.com/LTC3769
3769f
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