LT1913EDD#PBF Linear Technology, LT1913EDD#PBF Datasheet - Page 12

• Manufacturer Part Number: LT1913EDD#PBF
• Description: IC REG STEP DOWN 3.5A 10-DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1913EDD#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.79 ~ 25 V
• Current - Output: 3.5A
• Frequency - Switching: 200kHz ~ 2.4MHz
• Voltage - Input: 3.6 ~ 25 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-DFN
• Primary Input Voltage: 25V
• No. Of Outputs: 1
• Output Voltage: 25V
• Output Current: 3.5A
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS INFORMATION
LT1913
load is lower than 3.5A, then you can decrease the value
of the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher effi ciency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. 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 opera-
tion, see Linear Technology Application Note 44. Finally,
for duty cycles greater than 50% (V
is a minimum inductance required to avoid subharmonic
oscillations. See AN19.
Input Capacitor
Bypass the input of the LT1913 circuit with a ceramic
capacitor of X7R or X5R type. Y5V types have poor
performance over temperature and applied voltage, and
should not be used. A 10μF to 22μF ceramic capacitor is
adequate to bypass the LT1913 and will easily handle the
ripple current. Note that larger input capacitance is required
when a lower switching frequency is used. If the input
power source has high impedance, or there is signifi cant
inductance due to long wires or cables, additional bulk
capacitance may be necessary. This can be provided with
a lower performance electrolytic capacitor.
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 LT1913 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 10μF capacitor is capable of this task, but only if it is
placed close to the LT1913 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT1913. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT1913 circuit
12
OUT
/V
IN
> 0.5), there
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT1913’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 4.7μF ceramic capaci-
tor can be used for local bypassing of the LT1913 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT1913 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated by the
LT1913 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT1913’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
where f
output capacitance in μF . Use X5R or X7R types. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with a
higher value capacitor if the compensation network is also
adjusted to maintain the loop bandwidth. A lower value
of output capacitor can be used to save space and cost
but transient performance will suffer. See the Frequency
Compensation section to choose an appropriate compen-
sation network.
When choosing a capacitor, look carefully through the
data sheet to fi nd out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
C
OUT
SW
=
V
is in MHz, and C
OUT
100
f
SW
OUT
is the recommended
1913f