LTC3709 Linear Technology, LTC3709 Datasheet - Page 15

• Manufacturer Part Number: LTC3709
• Description: No RSENSE Synchronous DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
In the Figure 2 graph, the local maximum input RMS
capacitor currents are reached when:
These worst-case conditions are commonly used for
design because even significant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based on only 2000 hours of life
which makes it advisable to derate the capacitor. Several
capacitors may also be paralleled to meet size or height
requirements in the design. Always consult the capacitor
manufacturer if there is any question.
It is important to note that the efficiency loss is propor-
tional to the input RMS current squared and therefore a
2-stage implementation results in 75% less power loss
when compared to a single phase design. Battery/input
protection fuse resistance (if used), PC board trace and
connector resistance losses are also reduced by the re-
duction of the input ripple current in a 2-phase system. The
required amount of input capacitance is further reduced by
the factor 2 due to the effective increase in the frequency
of the current pulses.
V
V
OUT
IN
=
Figure 2. RMS Input Current Comparison
0.6
0.5
0.4
0.3
0.2
0.1
2
0
k
0.1
4
–
1
0.2
where k 1, 2
U
0.3
DUTY FACTOR (V
0.4
U
1-PHASE
2-PHASE
0.5
=
OUT
0.6
/V
W
IN
0.7
)
0.8
3709 F02
0.9
U
The selection of C
required to minimize voltage ripple and load step transients.
The output ripple ∆V
Since ∆I
highest at maximum input voltage. Typically, once the ESR
requirement is satisfied, the capacitance is adequate for
filtering and has the necessary RMS current rating.
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. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important to only use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR, but
can be used in cost-sensitive applications providing that
consideration is given to ripple current ratings and long-
term reliability. Ceramic capacitors have excellent low
ESR characteristics but can have a high voltage coefficient
and audible piezoelectric effects. High performance
through-hole capacitors may also be used, but an addi-
tional ceramic capacitor in parallel is recommended to
reduce the effect of their lead inductance.
Top MOSFET Driver Supply (C
An external bootstrap capacitor C
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D
when the switch node is low. Note that the average voltage
across C
turns on, the switch node rises to V
rises to approximately V
∆
V
OUT
L
B
increases with input voltage, the output ripple is
is approximately DRV
≤
∆
I ESR
L
⎛
⎜
⎝
OUT
OUT
is primarily determined by the ESR
+
IN
8
is approximately bounded by:
fC
+ DRV
1
OUT
B
CC
B
⎞
⎟
⎠
, D
connected to the BOOST
CC
. When the top MOSFET
B
IN
. The boost capacitor
)
and the BOOST pin
LTC3709
B
from DRV
15
3709f
CC