|
LTC3850-1_15 Datasheet, PDF (29/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller | |||
|
◁ |
LTC3850/LTC3850-1
APPLICATIONS INFORMATION
The Coiltronics (Cooper) HCP0703-2R2 (20mΩ DCRMAX
at 20°C) and HCP0703-3R3 (30m⦠DCRMAX at 20°C) are
chosen. At 100°C, the estimated maximum DCR values are
26.4m⦠and 39.6mâ¦. The divider ratios are:
RD
=
RSENSE(EQUIV )
DCRMAX at TL(MAX
)
=
7mâ¦
26.4mâ¦
= 0.26;
and
7mâ¦
39.6mâ¦
â
0.18
For each channel, 0.1µF is selected for C1.
R1||R2 =
(DCRMAX
L
at
20°C) ⢠C1 =
2.2µH
20m⦠⢠0.1µF
= 1.1k ;
and
3.3µH
30m⦠⢠0.1µF
=
1.1k
For channel 1, the DCRSENSE filter/divider values are:
R1= R1||R2 = 1.1k â
6.19k;
RD 0.18
R2
=
R1 ⢠RD
1â RD
=
6.19k ⢠0.18
1â 0.18
â
1.33k
The power loss in R1 at the maximum input voltage is:
PLOSSR1= (VIN(MAX)
â VOUT ) ⢠VOUT
R1
=
(20V
â 3.3V)
6.19k
â¢
3.3V
=
9mW
100
10
DCR
90
7mâ¦
80
1
DCR
70
60
0.1
50
40
0.01
EFFICIENCY
POWER LOSS
0.01
0.1
1
10
LOAD CURRENT (mA)
38501 F15
Figure 15. Design Example Efficiency vs Load
The respective values for Channel 2 are R1 = 4.12k, R2 =
1.5k; and PLOSS R1 = 8mW.
Burst Mode operation is chosen for high light load efficiency
(Figure 15) by floating the MODE/PLLIN pin. Power loss
due to the DCR sensing network is slightly higher at light
loads than would have been the case with a suitable sense
resistor (7mâ¦). At heavier loads, DCR sensing provides
higher efficiency.
The power dissipation on the topside MOSFET can be easily
estimated. Choosing a Siliconix Si4816BDY dual MOSFET
results in: RDS(ON) = 0.023â¦/0.016â¦, CMILLER @ 100pF.
At maximum input voltage with T(estimated) = 50°C:
PMAIN
=
3.3V
20V
(5)2
[1+
(0.005)(50°C
â
25°C)]
â¢
(0.023â¦)
+
(20V)2

ï£ï£¬
5A
2


(2â¦)(100pF
)
â¢


5
1
â 2.3
+
1
2.3


(500kHz
)
=
186mW
A short-circuit to ground will result in a folded back cur-
rent of:
ISC
=
(1/ 3)50mV
0.007â¦
â
1
2

ï£ï£¬
90ns(20V)
3.3µH 
=
2.1A
with a typical value of RDS(ON) and d = (0.005/°C)(20)
= 0.1. The resulting power dissipated in the bottom
MOSFET is:
PSYNC
=
20V â 3.3V
20V
(2.1A
)2
(1.125)(0.016â¦)
= 66mW
which is less than under full-load conditions.
CIN is chosen for an RMS current rating of at least 2A at
temperature assuming only channel 1 or 2 is on. COUT is
chosen with an ESR of 0.02⦠for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
VORIPPLE = RESR (âIL) = 0.02â¦(1.5A) = 30mVPâP
38501fc
29
|
▷ |