L5987 STMicroelectronics, L5987 Datasheet - Page 29

• 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

L5987
6.5
Thermal considerations
The thermal design is important to prevent the thermal shutdown of device if junction
temperature goes above 150 °C. The three different sources of losses within the device are:
Equation 30
Where D is the duty cycle of the application and the maximum
220 mΩ. Note that the duty cycle is theoretically given by the ratio between V
but actually it is quite higher to compensate the losses of the regulator. So the conduction
losses increases compared with the ideal case.
Equation 31
Where T
and the current flowing into it during turn ON and turn OFF phases, as shown in
T
switching time is 50ns.
Equation 32
where I
The junction temperature T
Equation 33
Where T
R
calculated as the parallel of many paths of heat conduction from the junction to the ambient.
For this device the path through the exposed pad is the one conducting the largest amount
SW
thJA
a)
b)
c)
is the equivalent switching time. For this device the typical value for the equivalent
is the equivalent thermal resistance junction to ambient of the device; it can be
Q
A
RISE
is the quiescent current (I
conduction losses due to the not negligible
equal to:
switching losses due to power MOSFET turn ON and OFF; these can be
calculated as:
Quiescent current losses, calculated as:
is the ambient temperature and P
and T
P
SW
FALL
=
V
IN
are the overlap times of the voltage across the power switch (V
⋅
I
J
OUT
can be calculated as:
Doc ID 14972 Rev 3
⋅
(
------------------------------------------ - Fsw
P
T
ON
T
Q
RISE
J
= 2.4 mA).
=
=
P
+
2
T
R
Q
T
A
DSON
FALL
+
=
TOT
Rth
V
IN
)
⋅
is the sum of the power losses just seen.
⋅
JA
(
⋅
I
OUT
I
Q
⋅
P
R
TOT
)
=
2
DS(on)
⋅
V
D
IN
⋅
of the power switch; these are
I
OUT
R
DS(on)
⋅
Application information
T
SW
over temperature is
⋅
F
SW
OUT
Figure
and V
29/41
DS
IN
17.
,
)