Datasheet

LTC3833
26
3833f
APPLICATIONS INFORMATION
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current there is no change in efficiency.
Power losses in the switching regulator will reflect as a
longer than ideal on-time. This efficiency accounted on-
time in continuous mode can be calculated as:
t
ON(REAL)
≈
t
ON(IDEAL)
Efficiency
Design Example
Consider a step-down converter with V
IN
= 6V to 24V, V
OUT
= 1.2V, I
OUT(MAX)
= 15A, and f = 350kHz (see Figure 9).
The regulated output voltage is determined by:
V
OUT
= 0.6V • 1+
R
FB2
R
FB1






Using a 20k resistor from V
OSNS
+
to V
OSNS
–
, the top
feedback resistor is also 20k.
The frequency is programmed by:
R
T
kΩ
[ ]
=
41550
f kHz
[ ]
– 2.2 =
41550
350
– 2.2 ≈ 116.5k
Select the nearest standard value of 115k.
The minimum on-time occurs for maximum V
IN
and should
be greater than 20ns which is the best that the LTC3833
can achieve. The minimum on-time for this application is:
t
ON(MIN)
=
V
OUT
V
IN(MAX)
• f
=
1.2V
24V • 350kHz
≈ 143ns
Set the inductor value to give 40% ripple current at maxi-
mum V
IN
:
L =
1.2V
350kHz • 40% • 15A
• 1–
1.2V
24V






≈ 0.54µH
Select 0.56μH which is the nearest standard value.
V
IN
V
OUT
PGOOD
RUN
V
RNG
SENSE
–
SENSE
+
R
PGD
100k
INTV
CC
R
ITH
47.5k
R
T
115k
R
DCR
3.09k
LTC3833
R
FB2
20k
R
FB1
20k
C
OUT1
330µF
2.5V
×2
MT
MB
C
OUT2
100µF
×2
L1
0.56µH
TG
SW
D
B
C
SS
0.1µF
R
DIV1
52.3k
R
DIV2
10k
C
ITH1
470pF
C
ITH2
47pF
3833 F09
V
IN
6V TO 24V
V
OUT
1.2V
15A
C
DCR
0.1µF
C
B
0.1µF
C
VCC
4.7µF
C
IN1
: SANYO 25SVPD82M
C
OUT1
: SANYO 2R5TPE330M9
D
B
: CENTRAL CMDSH-3
L1: VISHAY IHLP4040DZ-056µH
MB: RENESAS RJK0330DPB
MT: RENESAS RJK0305DPB
TRACK/SS
ITH
RT
MODE/PLLIN
EXTV
CC
SGND
BOOST
INTV
CC
INTV
CC
BG
PGND
V
OSNS
+
V
OSNS
–
C
IN2
10µF
C
IN1
82µF
25V
+
+
350kHz
Figure 9. 1.2V, 15A, 350kHz Step-Down Converter
Efficiency
LOAD CURRENT (A)
0.1
60
EFFICIENCY (%)
80
100
1 10 100
3833 F09b
40
50
70
90
30
20
V
IN
= 12V
V
OUT
= 1.2V
PULSE-SKIPPING
MODE
FORCED
CONTINUOUS
MODE