LT1956IGN-5#TRPBF Linear Technology, LT1956IGN-5#TRPBF Datasheet - Page 14

• Manufacturer Part Number: LT1956IGN-5#TRPBF
• Description: IC SW REG STP-DWN HI VOLT 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1956IGN-5#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 1.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 5.5 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT1956/LT1956-5
APPLICATIO S I FOR ATIO
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
output 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
R
the V
1A using a 22 F ceramic output capacitor and V
ponent values of C
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 LT1956 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 LT1956, a larger value may be required. This
14
C
/C
I
I
RIPPLE RMS
RIPPLE RMS IN
C
C
network is suggested to control possible ripple at
pin. The LT1956 can be stabilized for V
(
(
)
)
C
0 29
C
U
.
= 4700pF, R
C
I
OUT
compensation capacitor C
V
OUT
U
L f V
V
OUT
V
IN
C
IN
= 4.7k and C
V
W
–
F
V
IN
IN
V
in parallel with the
OUT
–
2
V
C
OUT
can be placed
OUT
U
F
= 220pF.
C
C
= 5V at
. Care
com-
is to prevent excessive ripple causing dips below the mini-
mum operating voltage resulting in erratic operation.
Depending on how the LT1956 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 LT1956 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 LT1956.
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.
If tantalum capacitors are used, values in the 22 F to
470 F range are generally needed to minimize ESR and
meet ripple current and surge ratings. Care should be
taken to ensure the ripple and surge ratings are not
exceeded. The AVX TPS and Kemet T495 series are surge
rated. AVX recommends derating capacitor operating
voltage by 2 for high surge applications.
CATCH DIODE
Highest efficiency operation requires the use of a Schottky
type diode. DC switching losses are minimized due to its
low forward voltage drop, and AC behavior is benign due
to its lack of a significant reverse recovery time. Schottky
diodes are generally available with reverse voltage ratings
of up to 60V and even 100V, and are price competitive with
other types.
The use of so-called “ultrafast” recovery diodes is gener-
ally not recommended. When operating in continuous
mode, the reverse recovery time exhibited by “ultrafast”
diodes will result in a slingshot type effect. The power
1956f