LT3431EFE Linear Technology, LT3431EFE Datasheet - Page 14

• Manufacturer Part Number: LT3431EFE
• Description: IC SW REG STEP-DOWN 3A 16TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3431EFE

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.2 ~ 48 V
• Current - Output: 3A
• Frequency - Switching: 500kHz
• Voltage - Input: 5.5 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LT3431
APPLICATIO S I FOR ATIO
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true, and type TPS capacitors are
specially tested for surge capability, but surge ruggedness
is not a critical issue with the output capacitor. Solid
tantalum capacitors fail during very high turn-on surges,
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
triangular with a typical value of 250mA
to calculate this is:
Output capacitor ripple current (RMS):
Ceramic Capacitors
Ceramic capacitors are generally chosen for their good
high frequency operation, small size and very low ESR
(effective series resistance). Their low ESR reduces out-
put ripple voltage but also removes a useful zero in the
loop frequency response, common to tantalum capaci-
tors. To compensate for this, a resistor R
in series with the V
must be taken however, since this resistor sets the high
frequency gain of the error amplifier, including the gain at
the switching frequency. If the gain of the error amplifier
is high enough at the switching frequency, output ripple
voltage (although smaller for a ceramic output capacitor)
may still affect the proper operation of the regulator. A
filter capacitor C
suggested to control possible ripple at the V
Ceramic” solution is possible for the LT3431 by choosing
the correct compensation components for the given
application.
Example: For V
can be stabilized, provide good transient response and
maintain very low output ripple voltage using the follow-
ing component values: (refer to the first page of this data
14
I
RIPPLE RMS
IN
= 8V to 20V, V
F
0 29
in parallel with the R
U
.
C
compensation capacitor C
V
OUT
U
L f V
OUT
V
IN
IN
= 5V at 2A, the LT3431
W
V
OUT
RMS
C
C
/C
can be placed
. The formula
C
C
pin. An “All
network is
U
C
. Care
sheet for component references) C3 = 2.2 F, R
C
Note 19 for further detail on techniques for proper loop
compensation.
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple this causes at the input of LT3431 and force the
switching current into a tight local loop, thereby minimiz-
ing EMI. The RMS ripple current can be calculated from:
Ceramic capacitors are ideal for input bypassing. At 500kHz
switching frequency, the energy storage requirement of
the input capacitor suggests that values in the range of
2.2 F to 10 F are suitable for most applications. If opera-
tion is required close to the minimum input required by the
output of the LT3431, a larger value may be required. This
is to prevent excessive ripple causing dips below the
minimum operating voltage resulting in erratic operation.
Depending on how the LT3431 circuit is powered up you
may need to check for input voltage transients.
The input voltage transients may be caused by input
voltage steps or by connecting the LT3431 converter to an
already powered up source such as a wall adapter. The
sudden application of input voltage will cause a large
surge of current in the input leads that will store energy in
the parasitic inductance of the leads. This energy will
cause the input voltage to swing above the DC level of input
power source and it may exceed the maximum voltage
rating of input capacitor and LT3431.
The easiest way to suppress input voltage transients is to
add a small aluminum electrolytic capacitor in parallel with
the low ESR input capacitor. The selected capacitor needs
to have the right amount of ESR in order to critically
dampen the resonant circuit formed by the input lead
inductance and the input capacitor. The typical values of
ESR will fall in the range of 0.5 to 2 and capacitance will
fall in the range of 5 F to 50 F.
C
I
= 15nF, C
RIPPLE RMS
F
= 220pF and C1 = 47 F. See Application
I
OUT
V
OUT
V
IN
–
V
OUT
/
V
IN
2
C
sn3431 3431fs
= 1.5k,