L5981 STMicroelectronics, L5981 Datasheet - Page 14

• Manufacturer Part Number: L5981
• Description: IC REG SW STEP DOWN 1A 8-VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5981

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 18 V
• Current - Output: 1A
• Frequency - Switching: 250kHz ~ 1MHz
• Voltage - Input: 2.9 ~ 18 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, 8-VFQFPN
• Power - Output: 1.5W
• Output Voltage: 3 V
• Output Current: 1 A
• Input Voltage: 2.9 V to 18 V
• Switching Frequency: 220 KHz to 275 KHz
• Operating Temperature Range: - 40 C to + 150 C
• Mounting Style: SMD/SMT
• Duty Cycle (max): 100 %
• Low Dropout Operation: 100 % duty cycle
• For Use With: 497-6385 - BOARD EVAL FOR L5981
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application information
5
5.1
14/37
Application information
Input capacitor selection
The capacitor connected to the input has to be capable to support the maximum input
operating voltage and the maximum RMS input current required by the device. The input
capacitor is subject to a pulsed current, the RMS value of which is dissipated over its ESR,
affecting the overall system efficiency.
So the input capacitor must have a RMS current rating higher than the maximum RMS input
current and an ESR value compliant with the expected efficiency.
The maximum RMS input current flowing through the capacitor can be calculated as:
Equation 3
Where Io is the maximum DC output current, D is the duty cycle, η is the efficiency.
Considering
In a specific application the range of possible duty cycles has to be considered in order to
find out the maximum RMS input current. The maximum and minimum duty cycles can be
calculated as:
Equation 4
and
Equation 5
Where V
the internal PDMOS. In
are reported
Table 6.
Manufacturer
MURATA
F
TDK
is the forward voltage on the freewheeling diode and V
η
Input MLCC capacitors
= 1, This function has a maximum at D = 0.5 and it is equal to Io/2.
Table 6
Doc ID 13004 Rev 6
GRM31
GRM55
Series
C3225
some multi layer ceramic capacitors suitable for this device
I
RMS
D
D
MAX
MIN
=
I
=
O
=
⋅
------------------------------------- -
V
------------------------------------ -
V
INMAX
V
V
INMIN
D
OUT
OUT
–
2 D
-------------- -
+
Cap value (μF)
⋅
+
–
–
η
V
V
V
V
2
F
SW
SW
F
+
D
------ -
η2
10
10
10
2
SW
is voltage drop across
Rated voltage (V)
25
25
25
L5981