LTC3864 Datasheet, PDF (21/28 Pages) Linear Technology – 60V Low IQ Step-Down DC/DC Controller with 100% Duty Cycle Capability

LTC3864
APPLICATIONS INFORMATION
The FREQ pin is tied to signal ground in order to program
the switching frequency to 350kHz. The on-time required
at 55V to generate a 5V output can be calculated as:
t_ON
=
V_OUT
V_IN • f
=
55V
5V
• 350kHz
≈
260ns
This on-time is larger than LTC3864’s minimum on-time
with sufficient margin to prevent cycle skipping.
Next, set the inductor value to give 60% worst-case ripple
at maximum V_IN = 55V.
L=

5V
_{350kHz •(0.6}
•
2A)
_1–
5V ^
55V
≈ 10.8µH
Select 10µH, which is a standard value.
The resulting maximum ripple current is:
∆I_L=

5V
350kHz •
_1–
10µH 
5V
55V
≈

1.3A
Next, set the R_SENSE resistor value to ensure that the
converter can deliver a maximum output current of 2.0A
with sufficient margin to account for component varia-
tions and worst-case operating conditions. Using a 30%
margin factor:
R_SENSE
≅
95mV
1.3
•


2A
+
1.3A
2


≈
27.5mΩ
Use a more readily available 25mΩ sense resistor.
The current limit is:
I_LIMIT
≅
95mV
25mΩ
–
1.3A
2
≈
3.15A
Next choose a P-channel MOSFET with the appropri-
ate BV_DSS and I_D rating. In this example, a good choice
is the Fairchild FDMC5614P (BV_DSS = 60V, I_D = 5.7A,
R_DS(ON) = 105mΩ, ρ_100°C = 1.5, C_MILLER = 100pF,
θ_JA = 60°C/W). The expected power dissipation and the
R_RUN
100k
C_SS
0.1µF
C_ITH1
3.3nF _{RITH 9.53k}
C_ITH2 100pF
C_CAP
0.1µF
CAP
RUN
V_IN
MODE/PLLN
SS
SENSE
LTC3864
GATE
ITH
FREQ
SGND
PGOOD
VFB
PGND
C_IN2 ⁺
4.7µF
V_IN*
C_IN1 ^{5.2V TO 55V}
12µF
63V
C_VIN
0.1µF
R_SENSE
25mΩ
MP L1
10µH
SW
D1
R_PGD
100k
C_FF
47pF
R_FB2
422k
R_FB1
80.6k
C_IN1: NICHICON UPJ1J120MDD
D1: DIODES INC SBR3U100LP
L1: TOKO 1217AS-H-100M
MP: FAIRCHILD FDMC5614P
*V_OUT FOLLOWS V_IN WHEN 3.5V ≤ V_IN ≤ 5.2V
SEE DROPOUT BEHAVIOR IN TYPICAL PERFORMANCE CHARACTERISTICS
47µF
×2
V_OUT*
5V
2A
3864 F08a
Efficiency
100
Burst Mode
90 ^OPERATION
80
PULSE-SKIPPING
70
60
50
0.01
V_IN = 12V
V_OUT = 5V
0.1
1
LOAD CURRENT (A)
3864 F08b
Figure 8. Design Example (5V, 2A 350kHz Step-Down Converter)
3864f
21