LT3513EUHF#PBF Linear Technology, LT3513EUHF#PBF Datasheet - Page 14

• Manufacturer Part Number: LT3513EUHF#PBF
• Description: IC REG 5-OUT FOR TFT-LCD 38QFN
• Manufacturer: Linear Technology

Specifications of LT3513EUHF#PBF

• Applications: Converter, TFT, LCD
• Voltage - Input: 4.5 ~ 30 V
• Number Of Outputs: 5
• Voltage - Output: 0.8 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 38-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

operaTion
LT3513
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3513 limits its switch cur-
rent in order to protect itself and the system from overload
faults. Therefore, the maximum output current that the
LT3513 will deliver depends on the switch current limit, the
inductor value, and the input and output voltages. When
the switch is off, the potential across the inductor is the
output voltage plus the catch diode drop. This gives the
peak-to-peak ripple current in the inductor:
where f is the switching frequency of the LT3513 and L
is the value of the inductor. The peak inductor and switch
current is:
To maintain output regulation, this peak current must be
less than the LT3513’s switch current limit of I
I
1.5A at DC = 0.75 as shown in the Typical Performance
Characteristics section. The maximum output current is
a function of the chosen inductor value:
Choosing an inductor value so that the ripple current is
small will allow a maximum output current 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 LT3513 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
is less than ΔI
14
LIM
I
I
∆I
OUT(MAX)
SW(PK)
is at least 2A at DC = 0.35, and decreases linearly to
L
=
(
1– Dc
=I
=I
LPK
L
/2.
LIM
)
L • f
(
=I
V
–
OUT
OUT
∆I
2
L
+ V
+
= 2.5A • 1– 0.57 • Dc
∆I
F
2
L
)
(
LIM
. For SW1,
)
–
∆I
2
OUT
L
OUTPUT CAPACITOR SELECTION
For 5V and 3.3V outputs, a 10µF 6.3V ceramic capacitor
(X5R or X7R) at the output results in very low output volt-
age ripple and good transient response. Other types and
values will also work; the following discussion explores
tradeoffs in output ripple and transient performance.
The output capacitor filters the inductor current to gen-
erate an output with low voltage ripple. It also stores
energy in order satisfy transient loads and stabilizes the
LT3513’s control loop. Because the LT3513 operates at a
high frequency, minimal output capacitance is necessary.
In addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
You can estimate output ripple with the following
equations:
where ΔI
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
V
and aluminum)
I
c
V
c(RMS)
RIPPLE
RIPPLE
OUT
L
> 10 • L •
is the peak-to-peak ripple current in the inductor.
= ΔI
=
=
8 • f • c
∆I
L
12
L
• ESR for electrolytic capacitors (tantalum
∆I

 
L
V
I
OUT
LIM
OUT

 
for ceramic capacitors, and
2
3513fc