LTC3786_15 Datasheet, PDF (17/34 Pages) Linear Technology – Low IQ Synchronous Boost Controller

LTC3786
Applications Information
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 + δ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs Temperature curve, but
δ = 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, volt-
age 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 the 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 choos-
ing 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:
( ) _VRIPPLE
=
I_OUT(MAX) •
C_OUT
V_OUT – V_IN(MIN)
• V_OUT • 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 (i.e., OS-CON and POSCAP).
Setting Output Voltage
The LTC3786 output voltage is set by an external feedback
resistor divider carefully placed across the output, as shown
in Figure 3. The regulated output voltage is determined by:
V_OUT
=

1.2V _1+

R_B
R_A



Great care should be taken to route the VFB line away
from noise sources, such as the inductor or the SW line.
Also, keep the VFB node as small as possible to avoid
noise pickup.
V_OUT
LTC3786
R_B
VFB
R_A
3786 F03
Figure 3. Setting Output Voltage
3786fa
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