ISL6537A Intersil Corporation, ISL6537A Datasheet - Page 14

• Manufacturer Part Number: ISL6537A
• Description: ACPI Regulator/Controller
• Manufacturer: Intersil Corporation

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For a through hole design, several electrolytic capacitors
may be needed. For surface mount designs, solid tantalum
capacitors can be used, but caution must be exercised with
regard to the capacitor surge current rating. These
capacitors must be capable of handling the surge-current at
power-up. Some capacitor series available from reputable
manufacturers are surge current tested.
MOSFET Selection - PWM Buck Converter
The ISL6537A requires 2 N-Channel power MOSFETs for
switching power and a third MOSFET to block backfeed from
V
based upon r
management requirements.
In high-current applications, the MOSFET power dissipation,
package selection and heatsink are the dominant design
factors. The power dissipation includes two loss
components; conduction loss and switching loss. The
conduction losses are the largest component of power
dissipation for both the upper and the lower MOSFETs.
These losses are distributed between the two MOSFETs
according to duty factor. The switching losses seen when
sourcing current will be different from the switching losses
seen when sinking current. When sourcing current, the
upper MOSFET realizes most of the switching losses. The
lower switch realizes most of the switching losses when the
converter is sinking current (see the equations below).
These equations assume linear voltage-current transitions
and do not adequately model power loss due the reverse-
recovery of the upper and lower MOSFET’s body diode. The
gate-charge losses are dissipated in part by the ISL6537A
and do not significantly heat the MOSFETs. However, large
gate-charge increases the switching interval, t
increases the MOSFET switching losses. Ensure that both
MOSFETs are within their maximum junction temperature at
high ambient temperature by calculating the temperature
rise according to package thermal-resistance specifications.
A separate heatsink may be necessary depending upon
MOSFET power, package type, ambient temperature and air
flow.
Approximate Losses while Sinking current
Approximate Losses while Sourcing current
DDQ
P
P
P
P
LOWER
LOWER
UPPER
UPPER
Where: D is the duty cycle = V
to the Input in S3 Mode. These should be selected
= Io
= Io
=
=
t
f
SW
s
DS(ON)
Io
Io
2
is the switching frequency.
2
2
x r
2
x r
is the combined switch ON and OFF time, and
×
×
DS(ON)
DS(ON)
r
r
DS ON
DS ON
, gate supply requirements, and thermal
(
(
x D
x (1 - D)
)
)
×
×
D
(
14
1
+
∠
1
-- - Io
2
OUT
D
⋅
)
+
/V
×
1
-- - Io
2
IN
V
⋅
,
IN
×
×
t
V
SW
IN
SW
×
×
which
t
f
SW
s
×
f
s
ISL6537A
MOSFET Selection - LDO
The main criteria for selection of the linear regulator pass
transistor is package selection for efficient removal of heat.
Select a package and heatsink that maintains the junction
temperature below the rating with a maximum expected
ambient temperature.
The power dissipated in the linear regulator is:
where I
nominal output voltage of the linear regulator.
P
LINEAR
O
≅
is the maximum output current and V
I
O
×
(
V
IN
∠
V
OUT
)
OUT
is the
FN9143.3