LTC3862_15 Datasheet, PDF (33/42 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
Applications Information
10. The inductor ripple current was chosen to be 40%
and the maximum load current is 5A. For a current
limit set at 30% above the maximum load current, the
maximum switch and sense resistor currents are:
I_SW(MAX)
=
I_R(SENSE)
=
1
n
•


1+
χ
2 
•
1.3 • I_O(MAX)
1– D_MAX
=
1
2
•


1+
0.4
2 
•
1.3 • 5A
1– 0.505
=
7.9A
11. The maximum current sense threshold for the LTC3862
is 75mV at low duty cycle and a normalized slope gain
of 1.0. Using Figure 21, the maximum sense voltage
drops to 73mV at a duty cycle of 51% with a normalized
slope gain of 1, so the sense resistor is calculated to
be:
R_SENSE

=
V_SENSE(MAX)
I_SW(MAX)
=
73mV
7.9A
=
9.2mΩ
For this application a 8mΩ, 1W surface mount resistor
was used for each phase.
12. The power dissipated in the sense resistors in current
limit is:
( ) ^PR(SENSE)
=



1.3 • I_O(MAX)
n • 1– D_MAX
²


• R_SENSE
• D_MAX
( ) ^
=


2
•
1.3 • 5
1– 0.505
²
 ^{• 0.009 • 0.505}

= 0.20W
13. The average current in the boost diodes is half the
output current (5A/2 = 2.5A), but the peak current in
each diode is:
The diode chosen for this application is the 30BQ060,
manufactured by International Rectifier. This surface
mount diode has a maximum average forward current
of 3A at 125°C and a maximum reverse voltage of 60V.
The maximum forward voltage drop at 25°C is 0.65V
and is 0.42V at 125°C (the positive TC of the series
resistance is compensated by the negative TC of the
diode forward voltage).
The power dissipated by the diode is approximately:
P_D = I_D(PEAK) • V_F(PEAK) • (1 – D_MAX)
= 6.06A • 0.42V • (1 – 0.505) = 1.26W
14. Two types of output capacitors are connected in parallel
for this application; a low ESR ceramic capacitor and
an aluminum electrolytic for bulk storage. For a 1%
contribution to the total ripple voltage, the maximum
ESR of the composite output capacitance is approxi-
mately:
ESR_COUT
≤
0.01• V_OUT
I_D(PEAK)
=
0.01• 48V
4.4A
= 0.109Ω
For the bulk capacitance, which we assume contributes
1% to the total output ripple, the minimum required
capacitance is approximately:
C_OUT
≥
I_O(MAX)
0.01• n • V_OUT
•
f
=
0.01•
2
•
5A
48V
•
300kHz
T
= 17.5µF
For this application, in order to obtain both low ESR
and an adequate ripple current rating (see Figure 24),
two 100μF, 63V aluminum electrolytic capacitors were
connected in parallel with four 6.8μF, 50V ceramic
capacitors. Figure 27 illustrates the switching wave-
forms for this application circuit.
I_D(PEAK)
=
1
n
•


1+
χ
2


•
I_O(MAX)
1– D_MAX
=
1
2
•


1+
0.4
2 
•
1–
5A
0.505
=
6.06A
For more information www.linear.com/LTC3862
3862fc
33