LT1943EFE#TR Linear Technology, LT1943EFE#TR Datasheet - Page 13

• Manufacturer Part Number: LT1943EFE#TR
• Description: IC REG SW QUAD TFT LCD 28-TSSOP
• Manufacturer: Linear Technology

Specifications of LT1943EFE#TR

• Applications: Converter, TFT, LCD
• Voltage - Input: 4.5 ~ 22 V
• Number Of Outputs: 4
• Voltage - Output: 1.25 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP Exposed Pad, 28-eTSSOP, 28-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

OPERATIO
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
voltage ripple and good transient response. Other types
and values will also work; the following discussion ex-
plores tradeoffs in output ripple and transient perfor-
mance.
The output capacitor filters the inductor current to gener-
ate an output with low voltage ripple. It also stores energy
in order satisfy transient loads and stabilizes the LT1943’s
control loop. Because the LT1943 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 ∆I
tor. 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:
The low ESR and small size of ceramic capacitors make
them the preferred type for LT1943 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V
and Z5U types lose a large fraction of their capacitance
with applied voltage and at temperature extremes.
V
V
and aluminum);
I
C
C(RMS)
RIPPLE
RIPPLE
OUT
> 10 • L • (I
L
= ∆I
is the peak-to-peak ripple current in the induc-
= ∆I
= ∆I
L
L
L
/√12
/(8 • f • C
• ESR for electrolytic capacitors (tantalum
U
LIM
/V
OUT
OUT
) for ceramic capacitors, and
)
2
Because loop stability and transient response depend on
the value of C
and X5R types.
Electrolytic capacitors are also an option. The ESRs of
most aluminum electrolytic capacitors are too large to
deliver low output ripple. Tantalum and newer, lower ESR
organic electrolytic capacitors intended for power supply
use are suitable, and the manufacturers will specify the
ESR. Chose a capacitor with a low enough ESR for the
required output ripple. Because the volume of the capaci-
tor determines its ESR, both the size and the value will be
larger than a ceramic capacitor that would give similar
ripple performance. One benefit is that the larger capaci-
tance may give better transient response for large changes
in load current. Table 2 lists several capacitor vendors.
Table 2. Low ESR Surface Mount Capacitors
Vendor
Taiyo Yuden
AVX
Kemet
Sanyo
Panasonic
TDK
Diode Selection
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode current
will then increase to the typical peak switch current.
Peak reverse voltage is equal to the regulator input volt-
age. Use a diode with a reverse voltage rating greater than
the input voltage. Table 3 lists several Schottky diodes and
their manufacturers.
I
D(AVG)
= I
OUT
OUT
, this loss may be unacceptable. Use X7R
(V
IN
Ta or Al Organic
Ta Organic
Al Organic
Al Organic
– V
Tantalum
Tantalum
Ceramic
Ceramic
Ceramic
Type
OUT
)/V
IN
T491, T494, T495
X5R, X7R
X5R, X7R
X5R, X7R
POSCAP
SP CAP
LT1943
Series
T520
A700
TPS
13
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