LTC3709 Datasheet, PDF (15/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

LTC3709
APPLICATIO S I FOR ATIO
In the Figure 2 graph, the local maximum input RMS
capacitor currents are reached when:
V_{OUT =} 2k – 1 _{where k = 1, 2}
V_IN
4
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.
0.6
0.5
0.4
1-PHASE
2-PHASE
0.3
0.2
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (V_OUT/V_IN)
3709 F02
Figure 2. RMS Input Current Comparison
The selection of C_OUT is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The output ripple ∆V_OUT is approximately bounded by:
∆V_OUT
≤
∆I_L ^{⎛⎝⎜}ESR
+
1
8fC_OUT
⎞
⎠⎟
Since ∆I_L increases with input voltage, the output ripple is
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_B, D_B)
An external bootstrap capacitor C_B connected to the BOOST
pin supplies the gate drive voltage for the topside MOSFET.
This capacitor is charged through diode D_B from DRV_CC
when the switch node is low. Note that the average voltage
across C_B is approximately DRV_CC. When the top MOSFET
turns on, the switch node rises to V_IN and the BOOST pin
rises to approximately V_IN + DRV_CC. The boost capacitor
3709f
15