LT3500 LINER [Linear Technology], LT3500 Datasheet - Page 14

• Manufacturer Part Number: LT3500
• Description: Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller
• Manufacturer: LINER [Linear Technology]

LT3500
To maintain output regulation, this peak current must be
less than the LT3500’s switch current limit, I
guaranteed to be greater than 2.3A over the entire duty
cycle range. The maximum output current is a function
of the chosen inductor value:
If the inductor value is chosen so that the ripple current
is small, then the available output current will be near the
switch current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors
and choose one to meet cost or space goals. Then use
these equations to check that the LT3500 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continuous.
Discontinuous operation occurs when I
I
Figure 4 illustrates the inductance value needed for a 3.3V
output with a maximum load capability of 2A. Referring
to Figure 4, an inductor value between 3.3μH and 4.7μH
will be suffi cient for a 15V input voltage and a switch
frequency of 750kHz. There are several graphs in the
Typical Performance Characteristics section of this data
sheet that show inductor selection as a function of input
voltage and switch frequency for several popular output
voltages and output ripple currents. Also, low inductance
may result in discontinuous mode operation, which is
okay, but further reduces maximum load current. For
details of maximum output current and discontinuous
mode operation, see Linear Technology Application Note
44. Finally, for duty cycles greater than 50% (V
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See Application Note 19 for
more information.
APPLICATIONS INFORMATION
14
L
/2 as calculated above.
I
OUT1(MAX)
=I
LIM
−
ΔI
2
L
=2.3 –
ΔI
2
L
OUT1
is less than
LIM
OUT1
. I
LIM
/V
is
IN
Input Capacitor Selection
Bypass the input of the LT3500 circuit with a 4.7μF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type can be used if there
is additional bypassing provided by bulk electrolytic or
tantalum capacitors. The following paragraphs describe
the input capacitor considerations in more detail.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3500 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
The input capacitor must have low impedance at the
switching frequency to do this effectively, and it must
have an adequate ripple current rating.
A conservative value is the RMS input current is given
by:
and is largest when V
I
CIN(RMS)
Figure 4. Inductor Values for 2A Maximum Load Current
(V
OUT1
2500
2000
1750
1500
1250
2250
1000
750
500
250
= 3.3V, I
=
5
I
OUT1
10
RIPPLE
L = 1μH
⎡ ⎣
15
V
IN
INPUT VOLTAGE (V)
OUT1
= 1A)
= 2V
20
• V
OUT1
(
V
25
IN
IN
− V
L = 1.5μH
L = 2.2μH
L = 3.3μH
L = 4.7μH
L = 6.8μH
30
(50% duty cycle).
OUT1
35
3500 F04
)
40
⎤ ⎦
0.5
<
I
OUT1
2
3500fb