LTC3862_15 Datasheet, PDF (29/42 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
Applications Information
case check the diode manufacturer’s data sheet to ensure
that its peak current rating exceeds the peak current in
the equation above. In addition, when calculating the
power dissipation in the diode, use the value of the for-
ward voltage (V_F) measured at the peak current, not the
average output current. Excess power will be dissipated
in the series resistance of the diode, which would not be
accounted for if the average output current and forward
voltage were used in the equations. Finally, this additional
power dissipation is important when deciding on a diode
current rating, package type, and method of heat sinking.
To a close approximation, the power dissipated by the
diode is:
P_D = I_D(PEAK) • V_F(PEAK) • (1 – D_MAX)
The diode junction temperature is:
T_J = T_A + P_D • R_TH(JA)
The R_TH(JA) to be used in this equation normally includes
the R_TH(JC) for the device plus the thermal resistance from
the board to the ambient temperature in the enclosure.
Once the proper diode has been selected and the circuit
performance has been verified, measure the temperature
of the power components using a thermal probe or infrared
camera over all operating conditions to ensure a good
thermal design.
Finally, remember to keep the diode lead lengths short
and to observe proper switch-node layout (see Board
Layout Checklist) to avoid excessive ringing and increased
dissipation.
Output Capacitor Selection
Contributions of ESR (equivalent series resistance), ESL
(equivalent series inductance) and the bulk capacitance
must be considered when choosing the correct combination
of output capacitors for a boost converter application. The
effects of these three parameters on the output voltage
ripple waveform are illustrated in Figure 23 for a typical
boost converter.
SW1
50V/DIV
SW2
50V/DIV
I_L1 2A/DIV
I_L2 2A/DIV
V_OUT
50mV/DIV
AC COUPLED
V_IN = 10V
V_OUT = 48V
500mA LOAD
1µs/DIV
3862 F23
Figure 23. Switching Waveforms for a Boost Converter
The choice of component(s) begins with the maximum
acceptable ripple voltage (expressed as a percentage of
the output voltage), and how this ripple should be divided
between the ESR step and the charging/discharging ∆V.
For the purpose of simplicity we will choose 2% for the
maximum output ripple, to be divided equally between the
ESR step and the charging/discharging ΔV. This percentage
ripple will change, depending on the requirements of the
application, and the equations provided below can easily
be modified.
One of the key benefits of multi-phase operation is a re-
duction in the peak current supplied to the output capacitor
by the boost diodes. As a result, the ESR requirement
of the capacitor is relaxed. For a 1% contribution to the
total ripple voltage, the ESR of the output capacitor can
be determined using the following equation:
ESR_COUT
≤
0.01• V_OUT
I_D(PEAK)
where:
I_D(PEAK)
=
1
n
•


1+
χ
2 
•
I_O(MAX)
1– D_MAX
The factor n represents the number of phases and the
factor χ represents the percentage inductor ripple current.
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3862fc
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