L5987 STMicroelectronics, L5987 Datasheet - Page 18

• Manufacturer Part Number: L5987
• Description: IC REG STEPDOWN 3A 8VFQFPN
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of L5987

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.6 V ~ 18 V
• Current - Output: 3A
• Frequency - Switching: 250kHz
• Voltage - Input: 2.9 V ~ 18 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 8-VFQFN, Exposed Pad
• 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 %
• Low Dropout Operation: 100 % duty cycle
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application information
6.2
18/41
Inductor selection
The inductance value fixes the current ripple flowing through the output capacitor. So the
minimum inductance value in order to have the expected current ripple has to be selected.
The rule to fix the current ripple value is to have a ripple at 20%-40% of the output current.
In the continuos current mode (CCM), the inductance value can be calculated by the
following equation:
Equation 10
Where T
time of the external diode (in CCM, F
fixed Vout, is obtained at maximum T
to calculate minimum duty). So fixing ΔI
minimum inductance value can be calculated:
Equation 11
where F
For example for V
inductance value to have ΔI
The peak current through the inductor is given by:
Equation 12
So if the inductor value decreases, the peak current (that has to be lower than the current
limit of the device) increases. The higher is the inductor value, the higher is the average
output current that can be delivered, without reaching the current limit.
In the table below some inductor part numbers are listed.
Table 7.
Manufacturer
SUMIDA
Coilcraft
SW
ON
Wurth
is the conduction time of the internal high side switch and T
is the switching frequency, 1/(T
Inductors
OUT
= 3.3 V, V
ΔI
CDRH6D226/HP
L
L
CDR10D48MN
= 30% of I
=
PD Type M
PD Type L
MSS1038
MSS1048
Doc ID 14972 Rev 3
Series
V
----------------------------- - T
IN
L
IN
MIN
= 12 V, I
–
L
OFF
V
SW
=
OUT
I
L PK
O
L
,
V
--------------------------- -
= 1/(T
that is at minimum duty cycle (see previous section
= 20% to 30% of the maximum output current, the
OUT
is about 10 μH.
ΔI
⋅
O
ON
MAX
=
= 3 A and F
ON
+
+ T
I
ON
O
V
=
Inductor value (μH)
+
F
OFF
+ T
⋅
ΔI
------- -
V
--------------------------- - T
2
1 D
---------------------- -
OUT
L
–
).
OFF
2.2 to 4.7
F
1.5 to 3.3
3.8 to 10
8.2 to 15
6.6 to 12
12 to 22
SW
L
+
MIN
SW
)). The maximum current ripple, at
V
F
= 250 kHz the minimum
⋅
OFF
Saturation current (A)
OFF
is the conduction
3.84 to 5.34
3.75 to 6.25
3.6 to 5.2
4.1 to 5.7
3.9 to 6.5
4 to 6
L5987