LTC3862 Datasheet, PDF (33/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
APPLICATIONS INFORMATION
The inductor value chosen was 18.7μH and the part
number is PB2020-223, manufactured by Pulse Engi-
neering. This inductor has a saturation current rating
of 20A.
8. The power MOSFET chosen for this application is
a Renesas HAT2266H. This MOSFET has a typical
R_DS(ON) of 11mΩ at V_GS = 4.5V and 9.2mΩ at V_GS
= 10V. The BV_DSS is rated at a minimum of 60V and
the maximum continuous drain current is 30A. The
typical gate charge is 25nC for a V_GS = 4.5V. Last but
not least, this MOSFET has an absolute maximum
avalanche energy rating EAS of 34mJ, indicating that it
is capable of avalanche without catastrophic failure.
9. The total IC quiescent current, IC power dissipation and
maximum junction temperature are approximately:
I_Q(TOT) = I_Q + 2 • Q_G(TOT) • f
= 3mA + 2 • 25nC • 300kHz = 18mA
P_DISS = 24V • 18mA = 432mW
T_J = 70°C + 432mW • 34°C/W = 84.7°C
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 20, the maximum sense voltage
drops to 73mV at a duty cycle of 51% with a normal-
ized 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:
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
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 paral-
lel 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 approximately:
ESR_COUT
≤
0.01• V_OUT
I_D(PEAK)
=
0.01• 48V
4.4A
= 0.109Ω
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