L5981 STMicroelectronics, L5981 Datasheet - Page 16

• 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
5.3
16/37
Output capacitor selection
The current in the capacitor has a triangular waveform which generates a voltage ripple
across it. This ripple is due to the capacitive component (charge and discharge of the output
capacitor) and the resistive component (due to the voltage drop across its ESR). So the
output capacitor has to be selected in order to have a voltage ripple compliant with the
application requirements.
The amount of the voltage ripple can be calculated starting from the current ripple obtained
by the inductor selection.
Equation 9
Usually the resistive component of the ripple is much higher than the capacitive one, if the
output capacitor adopted is not a multi layer ceramic capacitor (MLCC) with very low ESR
value.
The output capacitor is important also for loop stability: it fixes the double LC filter pole and
the zero due to its ESR. In
system stability.
For example with V
order to have a ΔV
needed and the ESR effect on the output voltage ripple can be neglected. In case of not
negligible ESR (electrolytic or tantalum capacitors), the capacitor is chosen taking into
account its ESR value.
So in case of 100 μF with ESR = 40 μF, the resistive component of the drop dominates and
the voltage ripple is 12 mV.
The output capacitor is also important to sustain the output voltage when a load transient
with high slew rate is required by the load. When the load transient slew rate exceeds the
system bandwidth the output capacitor provides the current to the load. So if the high slew
rate load transient is required by the application the output capacitor and system bandwidth
have to be chosen in order to sustain the load transient .
In the table below some capacitor series are listed.
Table 8.
Manufacturer
PANASONIC
MURATA
SANYO
TDK
Output capacitors
OUT
OUT
TPA/B/C
= 0.01·V
GRM32
GRM31
EEFCD
= 3.3 V, V
Series
C3225
ECJ
Chapter
ΔV
OUT
Doc ID 13004 Rev 6
OUT
IN
, if the multi layer capacitor are adopted, 10 μF are
=
= 12 V, ΔI
5.4, it will be illustrated how to consider its effect in the
ESR ΔI
Cap value (μF)
100 to 470
22 to 100
22 to 100
10 to 47
10 to 22
10 to 68
⋅
MAX
L
= 0.3 A (resulting by the inductor value), in
+
------------------------------------ -
8 C
⋅
ΔI
OUT
Rated voltage (V)
MAX
⋅
f
6.3 to 25
6.3 to 25
SW
4 to 16
6.3
6.3
6.3
ESR (mΩ)
15 to 55
40 to 80
< 5
< 5
< 5
< 5
L5981