isl6341a Intersil Corporation, isl6341a Datasheet - Page 15

• Manufacturer Part Number: isl6341a
• Description: 5v Or 12v Single Synchronous Buck Pulse-width Modulation Pwm Controller
• Manufacturer: Intersil Corporation

Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic
capacitors for high frequency decoupling and bulk capacitors
to supply the current needed each time Q
small ceramic capacitors physically close to the MOSFETs
and between the drain of Q
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and current
ratings above the maximum input voltage and largest RMS
current required by the circuit. The capacitor voltage rating
should be at least 1.25 times greater than the maximum
input voltage and a voltage rating of 1.5 times is a
conservative guideline. The RMS current rating requirement
for the input capacitor of a buck regulator is approximately
1/2 the DC load current.
For a through hole design, several electrolytic capacitors may
be needed. For surface mount designs, solid tantalum
capacitors can also 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/Considerations
The ISL6341, ISL6341A, ISL6341B requires 2 N-Channel
power MOSFETs. These should be selected based upon
r
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 Equation 12).
Equation 12 assumes linear voltage-current transitions and
does not adequately model power loss due to the reverse-
recovery of the upper and lower MOSFET’s body diode. The
gate-charge losses are dissipated by the ISL6341, ISL6341A,
ISL6341B and don't heat the MOSFETs. However, large gate-
charge increases the switching interval, t
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.
DS(ON)
, gate supply requirements, and thermal management
1
and the source of Q
15
SW
1
turns on. Place the
which increases
ISL6341, ISL6341A, ISL6341B
2
.
When operating with a 12V power supply for V
to a minimum supply voltage of 4.5V), a wide variety of
N-MOSFETs can be used. Check the absolute maximum
V
highest V
means a 20V V
30V V
(around 1V or below) are not recommended, for the reasons
explained in the following paragraph.
For 5V only operation, given the reduced available gate bias
voltage (5V), logic-level transistors should be used for both
N-MOSFETs. Look for r
should be exercised with devices exhibiting very low
V
present aboard the ISL6341, ISL6341A, ISL6341B may be
circumvented by these MOSFETs if they have large parasitic
impedences and/or capacitances that would inhibit the gate
of the MOSFET from being discharged below its threshold
level before the complementary MOSFET is turned on. Also
avoid MOSFETs with excessive switching times; the circuitry
is expecting transitions to occur in under 50ns or so.
BOOTSTRAP Considerations
Figure 15 shows the upper gate drive (BOOT pin) supplied
by a bootstrap circuit from V
usually shares the V
in the 5V to 12V range. The boot capacitor, C
develops a floating supply voltage referenced to the PHASE
pin. The supply is refreshed to a voltage of V
boot diode drop (V
turns on. Check that the voltage rating of the capacitor is
above the maximum V
should be sufficient for a 12V system. A value of 0.1µF is
typical for many systems driving single MOSFETs.
If V
option is to connect the BOOT pin to 12V, and remove the
BOOT cap (although, you may want to add a local cap from
BOOT to GND). This will make the UGATE V
equal to (12V - 5V = 7V). That should be high enough to
drive most MOSFETs, and low enough to improve the
efficiency slightly. This also saves a boot diode (and
capacitor).
Losses while Sinking Current
Losses while Sourcing Current
GS
GS(ON)
P
P
P
P
CC
UPPER
UPPER
LOWER
LOWER
rating for both MOSFETs; it needs to be above the
Where: D is the duty cycle = V
DS
is 12V, but V
CC
characteristics. The shoot-through protection
maximum rating). Low threshold transistors
= Io
= Io
=
=
t
f
voltage allowed in the system; that usually
SW
SW
Io
2
Io
2
GS
2
x r
2
x r
is the combined switch ON and OFF time, and
is the switching frequency.
×
×
DS(ON)
D
DS(ON)
r
rating (which typically correlates with a
r
IN
DS ON
DS ON
) each time the lower MOSFET, Q
IN
CC
is lower (such as 5V), then another
(
(
or V
DS(ON)
x D
x (1 - D)
voltage in the system; a 16V rating
)
)
×
CC
×
GD
D
(
1 D
+
supply; it can be any voltage
. For convenience, V
ratings at 4.5V. Caution
–
1
-- - Io
2
OUT
⋅
)
+
/ V
×
1
-- - Io
2
V
⋅
IN
IN
,
×
×
V
t
GD
SW
GS
BOOT
IN
CC
×
×
less the
voltage
August 20, 2007
t
F
(or down
SW
,
S
GD
(EQ. 12)
FN6538.0
×
2
,
F
S