L5981 STMicroelectronics, L5981 Datasheet - Page 26

• 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
26/37
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 26
Where D is the duty cycle of the application and the maximum R
the duty cycle is theoretically given by the ratio between V
higher to compensate the losses of the regulator. So the conduction losses increases
compared with the ideal case.
Equation 27
Where T
and the current flowing into it during turn ON and turn OFF phases, as shown in
T
switching time is 50 ns.
Equation 28
where I
The junction temperature T
Equation 29
Where T
Rth
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
JA
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 R
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 13004 Rev 6
⋅
P
(
------------------------------------------ - Fsw
T
ON
T
Q
RISE
J
= 2.4 mA).
=
=
P
+
2
R
T
Q
T
DS on
A
FALL
+
=
TOT
(
Rth
V
)
IN
)
⋅
is the sum of the power losses just seen.
⋅
JA
(
⋅
I
I
OUT
Q
⋅
P
TOT
)
=
DS(on)
2
⋅
V
D
IN
OUT
⋅
of the power switch; these are
I
OUT
an V
DSON
⋅
T
IN
SW
, but actually it is quite
is 220 mΩ. Note that
⋅
F
SW
Figure
L5981
DS
16.
)