MAX8513 MAXIM [Maxim Integrated Products], MAX8513 Datasheet - Page 21

• Manufacturer Part Number: MAX8513
• Description: Wide-Input, High-Frequency, Triple-Output Supplies with Voltage Monitor and Power-On Reset
• Manufacturer: MAXIM [Maxim Integrated Products]

where I
rent capability determined by:
where R
tance (1.5Ω typ) and R
tance of the MOSFET (≈2Ω).
where V
The total power loss in Q1 is:
In addition to the losses above, allow approximately
20% more for additional losses due to MOSFET output
capacitances and Q2 body-diode reverse recovery
charge dissipated in Q1. This is not typically well-
defined in MOSFET data sheets. Refer to the MOSFET
data sheet for the thermal-resistance specification to
calculate the PC board area needed to maintain the
desired maximum operating junction temperature with
the above calculated power dissipations.
To reduce EMI caused by switching noise, add a 0.1µF
or larger ceramic capacitor from the high-side MOSFET
drain to the low-side MOSFET source or add resistors
in series with DH and DL to slow down the switching
transitions. However, adding series resistors with DH
and DL increases the power dissipation in the MOSFET
when it switches, so be sure this does not overheat the
MOSFET. The minimum load current must exceed the
high-side MOSFET’s maximum leakage current over
temperature if fault conditions are expected.
Fast switching transitions cause ringing because of res-
onating circuit parasitic inductance and capacitance at
the switching nodes. This high-frequency ringing
occurs at LX’s rising and falling transitions and can
interfere with circuit performance and generate EMI. To
dampen this ringing, a series-RC snubber circuit is
added across each switch. The following is the proce-
dure for selecting the value of the series-RC circuit:
Wide-Input, High-Frequency, Triple-Output Supplies
P
P
Q DR
Q SW
1
GATE
1
GS
DH
P
=
≈ V
Q
=
is the high-side MOSFET driver’s on-resis-
Q
is the average DH high driver output-cur-
1
V
GS
VL
IN
=
I
GATE
P
= 5V.
Q CC
×
______________________________________________________________________________________
×
1
I
OUT
V
with Voltage Monitor and Power-On Reset
=
GS
(
+
MOSFET Snubber Circuit
GATE
R
1
DH
P
×
×
Q SW
1
f
2 5 .
S
f
+
S
is the internal gate resis-
×
R
×
V
GATE
(
+
R
(
GATE
Q
P
Q DR
GS
R
1
)
I
GATE
GATE
+
+
Q
R
DH
GD
)
)
1) Connect a scope probe to measure V
2) Find the capacitor value (connected from LX to
The circuit parasitic capacitance (C
equal to 1/3rd the value of the added capacitance above.
The circuit parasitic inductance (L
The resistor for critical dampening (R
(2π × f
tailor the desired damping and the peak voltage excur-
sion. The capacitor (C
times the value of the C
loss of the snubber circuit is dissipated in the resistor
(P
where V
frequency. Choose an R
the specific application’s derating rule for the power
dissipation calculated.
The MAX8513/MAX8514 can provide foldback current
limit or constant current limit. Unless constant current-
limit operation is required, such as when driving a con-
stant current load, foldback current limit should be
implemented. Foldback current limit reduces the power
dissipation of external components under overload or
short-circuit conditions.
For foldback current limit, the current-limit threshold is
set by an external resistive-divider from V
GND (R17 and R18 of the Typical Applications Circuits).
This makes the voltage at ILIM a function of the internal
5µA current source and V
parator threshold is equal to V
compared with V
across the current-sense resistor or, for lossless sens-
ing, the voltage across the inductor. When V
exceeds the current-limit threshold, the high-side
MOSFET turns off and the low-side MOSFET turns on.
This allows for a current foldback feature that reduces
the current-limit threshold during a short circuit. This
makes the current threshold limit, when V
percentage of the current-limit threshold, when V
at its nominal regulated value.
RSNUB
and observe the ringing frequency, f
GND) that reduces the ringing frequency by half.
R
× L
IN
) and can be calculated as:
PAR
is the input voltage and f
P
RSNUB
L
). Adjust the resistor value up or down to
PAR
SENSE
=
=
C
(
2
SNUB
SNUB
π
PAR
. V
SNUB
×
Foldback Current Limit
OUT1
SENSE
) should be at least 2 to 4
f
Current-Limit Setting
R
to be effective. The power
×
ILIM
)
1
2
power rating that meets
. The current-limit com-
( )
PAR
V
×
IN
/ 7.5. This threshold is
is either the voltage
C
) is calculated by:
2
PAR
PAR
S
SNUB
×
is the switching
R
OUT1
) at LX is then
f
.
S
OUT
) is equal to
LX
to ILIM to
to GND,
= 0V, a
OUT1
SENSE
21
is