LTC3769_15 Datasheet, PDF (18/32 Pages) Linear Technology – 60V Low IQ Synchronous Boost Controller

LTC3769
Applications Information
If the maximum output current is I_OUT(MAX) the MOSFET
power dissipation at maximum output current is given by:
P_MAIN = ^(VOUT
V_IN)V_OUT
V² _IN
• I_O2UT(MAX) • (1+
)
• R_DS(ON) + k • V_OUT3
•
I_OUT(MAX)
V_IN
• C_MILLER • f
P_SYNC
=
V_IN
V_OUT
• I_O2UT(MAX) • (1+
) •R_DS(ON)
where d is the temperature dependency of R_DS(ON). The
constant k, which accounts for the loss caused by reverse
recovery current, is inversely proportional to the gate drive
current and has an empirical value of 1.7.
Both MOSFETs have I²R losses while the bottom N-channel
equation includes an additional term for transition losses,
which are highest at low input voltages. For high V_IN the
high current efficiency generally improves with larger
MOSFETs, while for low V_IN the transition losses rapidly
increase to the point that the use of a higher R_DS(ON) device
with lower C_MILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the bottom switch duty factor is low or dur-
ing overvoltage when the synchronous switch is on close
to 100% of the period.
The term (1+ d) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs Temperature curve, but
d = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
C_IN and C_OUT Selection
The input ripple current in a boost converter is relatively
low (compared with the output ripple current), because this
current is continuous. The input capacitor C_IN voltage rating
should comfortably exceed the maximum input voltage.
Although ceramic capacitors can be relatively tolerant of
overvoltage conditions, aluminum electrolytic capacitors
are not. Be sure to characterize the input voltage for any
possible overvoltage transients that could apply excess
stress to the input capacitors.
The value of C_IN is a function of the source impedance, and
in general, the higher the source impedance, the higher the
required input capacitance. The required amount of input
capacitance is also greatly affected by the duty cycle. High
output current applications that also experience high duty
cycles can place great demands on the input supply, both
in terms of DC current and ripple current.
In a boost converter, the output has a discontinuous current,
so C_OUT must be capable of reducing the output voltage
ripple. The effects of ESR (equivalent series resistance) and
the bulk capacitance must be considered when choosing
the right capacitor for a given output ripple voltage. The
steady ripple voltage due to charging and discharging
the bulk capacitance in a single phase boost converter
is given by:
V_RIPPLE
=
I_OUT(MAX) •(V_OUT −
C_OUT • V_OUT
V_IN(MIN))
•f
V
where C_OUT is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
∆V_ESR = I_L(MAX) • ESR
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
Capacitors are now available with low ESR and high ripple
current ratings (e.g., OS-CON and POSCAP).
3769f
18
For more information www.linear.com/LTC3769