Datasheet

LT3500
12
3500fc
APPLICATIONS INFORMATION
Table 1. Effi ciency and Size Comparisons for Different R
RT/SYNC
Values, V
OUT1
= 3.3V
FREQUENCY R
T
/SYNC EFFICIENCY V
IN(MAX)
L C C + L AREA
(mm
2
)
2.5MHz 15k 73.6 12 1µ 10µ 24
2.0MHz 20k 81.5 14 1.5µ 10µ 24
1.5MHz 24.9k 84.5 18 2.2µ 10µ 24
1.0MHz 40.2k 87.3 28 3.3µ 22µ 34
500kHz 90.9k 88.9 36 4.7µ 47µ 40
The following example along with the data in Table 1
illustrates the tradeoffs of switch frequency selection.
Example.
V
IN
= 25V, V
OUT1
= 3.3V, I
OUT1
= 2.0A,
Temperature = 0°C to 85°C
t
ON(MIN)
= 185ns (85°C from Typical Characteris-
tics graph),
V
D
= 0.6V, V
SW
= 0.4V (85°C)
Max Frequency =
3.3 + 0.6
25 − 0.4 + 0.6
•
1
185ns
~ 835kHz
R
T
/SYNC ~ 49.9k
Frequency ≅ 820kHz
Input Voltage Range
Once the switching frequency has been determined, the
input voltage range of the regulator can be determined.
The minimum input voltage is determined by either the
LT3500’s minimum operating voltage of ~2.8V or by its
maximum duty cycle. The duty cycle is the fraction of time
that the internal switch is on during a clock cycle. The
maximum duty cycle can be determined from the clock
frequency and the minimum off time from the typical
characteristics graph.
This leads to a minimum input voltage of:
V
IN(MIN)
=
V
OUT1
+ V
D
DC
MAX
− V
D
+ V
SW
where V
SW
is the voltage drop of the internal switch,
and
DC
MAX
= 1 – t
OFF(MIN)
• Frequency.
Figure 3 shows a typical graph of minimum input voltage
vs load current for 3.3V and 5V applications.
The maximum input voltage is determined by the absolute
maximum ratings of the V
IN
and BST pins and by the
frequency and minimum duty cycle.
The minimum duty cycle is defi ned as:
DC
MIN
= t
ON(MIN)
• Frequency
Maximum input voltage as:
V
IN(MAX)
=
V
OUT1
+ V
D
DC
MIN
− V
D
+ V
SW
Figure 3. Minimum Input Voltage vs Load Current
LOAD CURRENT (A)
0
INPUT VOLTAGE (V)
3
4
5
1.0 1.6 1.8
3500 F03
2
0.2 0.4 0.6 0.8 1.41.2
6
7
8
2.0
V
OUT1
= 5V START-UP
V
OUT1
= 5V RUNNING
V
OUT1
= 3.3V START-UP
V
OUT1
= 3.3V RUNNING
f
SW
= 1MHz
L = 3.3µH