L5987TR STMicroelectronics, L5987TR Datasheet - Page 19

• Manufacturer Part Number: L5987TR
• Description: IC REG SW 3A STEPDOWN 8-VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5987TR

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 18 V
• Current - Output: 3A
• Frequency - Switching: 250kHz
• Voltage - Input: 2.9 ~ 18 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, 8-VFQFPN
• Power - Output: 1.5W
• Output Voltage: 0.6 V
• Output Current: 3 A
• Output Power: 1.5 W
• Input Voltage: 2.9 V to 18 V
• Switching Frequency: 250 KHz
• Mounting Style: SMD/SMT
• Duty Cycle (max): 100 %
• For Use With: 497-8369 - BOARD EVALUATION FOR L5987
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-8494-2

L5987
6.3
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 or 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 13
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
are 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 330 μF with ESR = 30 mΩ, the resistive component of
the drop dominates and the voltage ripple is 27 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
Doc ID 14972 Rev 3
OUT
OUT
IN
, if the multi layer ceramic capacitor are adopted, 13 μF
=
= 12 V, ΔI
6.4, 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
= 0.9 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
Application information
ESR (mΩ)
15 to 55
40 to 80
< 5
< 5
< 5
< 5
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