A5975DTR STMicroelectronics, A5975DTR Datasheet - Page 24

• Manufacturer Part Number: A5975DTR
• Description: IC REG SW STEP-DOWN 3A 8SOIC
• Manufacturer: STMicroelectronics
• Series: -r
• Type: Step-Down (Buck), PWMr

Specifications of A5975DTR

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.235 V ~ 35 V
• Current - Output: 3A
• Frequency - Switching: 250kHz
• Voltage - Input: 4 V ~ 36 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width) Exposed Pad
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 497-11426-2

Application information
8
8.1
24/50
Application information
Component selection
●
The input capacitor must be able to support the maximum input operating voltage and the
maximum RMS input current.
As step-down converters draw current from the input in pulses, the input current is squared
and the height of each pulse is equal to the output current. The input capacitor has to
absorb all this switching current, which can be up to the load current divided by two (worst
case, with duty cycle of 50%). For this reason, the quality of these capacitors must be very
high to minimize the power dissipation generated by the internal ESR, thereby improving
system reliability and efficiency. The critical parameter is usually the RMS current rating,
which must be higher than the RMS input current. The maximum RMS input current (flowing
through the input capacitor) is:
Equation 17
where η is the expected system efficiency, D is the duty cycle and I
current. This function reaches its maximum value at D = 0.5 and the equivalent RMS current
is equal to I
Equation 18
and
Equation 19
where V
internal PDMOS. Considering the range D
IRMS going through the input capacitor. Capacitors that can be considered are:
●
These are widely used due to their low price and their availability in a wide range of RMS
current ratings.
The only drawback is that, considering ripple current rating requirements, they are physically
larger than other capacitors.
●
If available for the required value and voltage rating, these capacitors usually have a higher
RMS current rating for a given physical dimension (due to very low ESR).
The drawback is the considerably high cost.
Input capacitor
Electrolytic capacitors:
Ceramic capacitors:
F
is the free-wheeling diode forward voltage and V
O
divided by 2 (considering η = 1). The maximum and minimum duty cycles are:
Doc ID 018760 Rev 1
I
RMS
D
D
MAX
MIN
=
=
I
=
O
------------------------------------ -
V
MIN
⋅
------------------------------------- -
V
INMIN
V
INMAX
V
D 2 D
OUT
to D
OUT
–
-------------- -
–
+
+
MAX
⋅
–
η
V
V
V
SW
V
F
2
F
SW
, it is possible to determine the max.
+
D
------ -
η
2
2
SW
the voltage drop across the
O
is the output DC
A5975D