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| LTC3611_15 Datasheet, PDF (18/26 Pages) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter | |||
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LTC3611
Applications Information
Design Example
As a design example, take a supply with the following
speciï¬cations: VIN = 5V to 36V (12V nominal), VOUT =
2.5V ±5%, IOUT(MAX) = 10A, f = 550kHz. First, calculate
the timing resistor with VON = VOUT:
RON
=
2.5V
(2.4)(550kHz)
(10pF)
=
187k
and choose the inductor for about 40% ripple current at
the maximum VIN:
L
=
2.5V
(550kHz)(0.4)(10A)
âââ
1â
2.5V
36V
ââ â
=
1µH
Selecting a standard value of 1μH results in a maximum
ripple current of:
ÎIL
=
2.5V
(550kHz)(1µH)
âââ
1â
2.5V
12V
ââ â
=
3.6A
Next, set up VRNG voltage and check the ILIMIT. Tying
VRNG to 1V will set the typical current limit to 15A, and
tying VRNG to GND will result in a typical current around
10A. CIN is chosen for an RMS current rating of about 5A
at 85°C. The output capacitors are chosen for a low ESR
of 0.013Ω to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
ÎVOUT(RIPPLE) = ÎIL(MAX) (ESR)
= (3.6A) (0.013Ω) = 47mV
However, a 0A to 10A load step will cause an output
change of up to:
ÎVOUT(STEP) = ÎILOAD (ESR) = (10A) (0.013Ω) =130mV
An optional 22μF ceramic output capacitor is included
to minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 6.
PGND
INTVCC
CVCC
4.7µF
6.3V
SW
CF
0.1µF
50V
64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49
VIN
SGND
VOUT
2.5V AT
10A
C5
COUT1
L1
1µH
22µF
100µF
6.3V
Ã2
(OPTIONAL)
GND
VIN
5V TO 32V
VIN
GND
CIN
4.7µF
C6 +
100µF
50V
50V
Ã2
(OPTIONAL)
1 PGND
2 PGND
3 PGND
4 SW
5 SW
6 SW
7 SW
8 SW
9 SW
10 SW
11 SW
12 PVIN
13 PVIN
14 PVIN
15 PVIN
16 PVIN
LTC3611
SGND 48
SGND 47
SGND 46
SGND 45
EXTVCC 44
VFB 43
SGND 42
ION 41
SGND 40
FCB 39
ITH 38
VRNG 37
PGOOD 36
VON 35
SGND 34
33
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
CIN = MURATA GRM32ER71H475K
COUT = MURATA GRM43SR60J107M
L1 = COOPER HCP0703-IRO
C5: MURATA GRM31CR60J226KE19
KEEP POWER AND SIGNAL GROUNDS SEPARATE.
CONNECT TO ONE POINT.
SW
2Ω
INTVCC
DB
CMDSH-3
CB1
0.22µF
SW
CSS
0.1µF
EXTVCC
C4
0.01µF
R1
9.5k
C1
1%
(OPTIONAL)
RON
182k
1%
CON
0.01µF
(OPTIONAL)
R5
12.5k
R3 39.2k
11k
RPG1
100k
INTVCC
RSS1
510k
VIN
(OPTIONAL)
0.01µF
R2
30.1k
1%
(OPTIONAL)
C2
VOUT
VIN
CC1
680pF
CC2
100pF
RVON
0Ω
VOUT
SGND
3611 F06
Figure 6. Design Example: 5V to 32V Input to 2.5V/10A at 550kHz
3611fd
18
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