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MSK5045-5.0HG Datasheet(PDF) 3 Page - M.S. Kennedy Corporation

Part # MSK5045-5.0HG
Description  HIGH EFFICIENCY, HIGH VOLTAGE 4 AMP SURFACE MOUNT SWITCHING REGULATORS
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Manufacturer  MSK [M.S. Kennedy Corporation]
Direct Link  http://www.mskennedy.com
Logo MSK - M.S. Kennedy Corporation

MSK5045-5.0HG Datasheet(HTML) 3 Page - M.S. Kennedy Corporation

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APPLICATION NOTES
SELECTING RS:
The MSK 5045 monitors the inductor current and the aver-
age load current by sensing the voltage across RS. Cycle-by-
cycle current limiting is controlled with an upper threshold of
100mV ±20mV; the high side MOSFET switch is gated off
whenever the upper threshold is exceeded. Pulse skipping oc-
curs in power save mode when the signal falls below the 30%
current threshold of 30mV. The sychronous rectifier is disabled
when the signal falls below 0V indicating discontinuous induc-
tor current. Selection of RS must take all of these features into
consideration.
When operated in the continuous conduction mode peak to
peak inductor current is approximated by the equation
(VIN-VOUT) VOUT
f L VIN
where f=300KHz and L=6.4µH. (If optional output inductance
is used L=6.4uH + optional L). The
device will operate in
continuous conduction as long as IOUT
≥ ½ Ip-p. The maxi-
mum and minimum current peaks are equal to IOUT±½ Ip-p.
RS translates the current levels into the control signal. Once
the current levels are established the designer can size RS for
specific applications. Care must be taken when selecting RS
because under a short circuit condition the output current will
approach the cycle-by-cycle current limit.
For most applications, it may be useful to wire the sense
inputs with a twisted pair instead of PCB traces. Low induc-
tance current sense resistors, such as metal film surface mount
styles are best.
Rev. G 2/06
3
.
.
.
POWER DISSIPATION:
In high current applications, it is very important to ensure
that both MOSFETS are within their maximum junction tem-
perature at high ambient temperatures. Temperature rise can
be calculated based on package thermal resistance and worst
case dissipation for each MOSFET. These worst case dissipa-
tions occur at minimum voltage for the high side MOSFET and
at maximum voltage for the low side MOSFET.
Calculate power dissipation using the following formulas:
Pd (upper FET)=ILOAD² x 0.090
Ω x DUTY
+ VIN x ILOAD x f x
VIN x CRSS+25ns
IGATE
Pd (lower FET)=ILOAD² x 0.090
Ω x (1-DUTY)
DUTY= (VOUT+VQ2)
(VIN-VQ1)
Where: VQ1 or VQ2 (on state voltage drop)=ILOAD x 0.090
CRSS=65pF
IGATE=2A
During output short circuit, Q2, the synchronous-rectifier
MOSFET, will have an increased duty factor and will see addi-
tional stress. This can be calculated by:
Q2 DUTY=1-
VQ2
VIN(MAX)-VQ1
Where: VQ1 or VQ2=(120MV/RSENSE) x 0.090
INPUT CAPACITOR SELECTION:
The MSK 5045 should have an external high frequency
ceramic capacitor (0.1uF) between VIN and GND. Connect a
low-ESR bulk capacitor directly to the input pin of the MSK
5045. Select the bulk input filter capacitor according to input
ripple-current requirements and voltage rating, rather than ca-
pacitor value.
Electrolytic capacitors that have low enough
ESR to meet the ripple-current requirement invariably have more
than adequate capacitance values. Aluminum-electrolytic ca-
pacitors are preferred over tantalum types, which could cause
power-up surge-current failure when connecting to robust AC
adapters or low-impedance batteries. RMS input ripple current
is determined by the input voltage and load current, with the
worst possible case occuring at VIN = 2 x VOUT:
IRMS = ILOAD x
√VOUT(VIN-VOUT)
VIN
INPUT BIAS AND UVLO:
Pin 11 of the MSK 5045 provides bias to an internal linear
regulator that powers the control circuitry. The Vbias pin can
be connected directly to the input bus for 12V to 80V opera-
tion or it can be biased separately with a 12V to 15V source to
extend the input range of the device and improve efficiency at
high line; refer to the paragraph titled "INPUT VOLTAGE
RANGE". Vbias must be applied simultaneous with or prior to
the input voltage. The MSK 5045's built in under voltage lock-
out feature prevents damage to downstream devices in the
event of a drop in bias voltage. Under voltage lockout occurs
at bias voltages of approximately 10V rising and 9.7V falling.
When separating the bias voltage from Vin to extend the input
range below the Vbias UVLO set point, a simple open collector
circuit can disable the device at any desired set point for Vin if
UVLO is required. The internal bias draws approximately 30mA
under normal operation and less than 10mA in Power Save
mode with a light load on the output.
INPUT VOLTAGE RANGE
The MSK 5045's wide input range of 12V to 80V can be
further extended down to VOUT + VDROPOUT by using a sepa-
rate bias supply; refer to the paragraph titled "LOW VOLTAGE
OPERATION". In this configuration very efficient low V to low
V conversion can be achieved. At high line voltages the inter-
nal linear regulator dissipates more power than at low line. This
loss in efficiency can be eliminated with a separate bias supply
pushing the high line efficiency up close to the low line perfor-
mance. Output ripple changes with line voltage; refer to the
paragraph titled "OUTPUT INDUCTOR (OPTIONAL)" for more
information.
SOFT START/Cton:
The internal soft-start circuitry allows a gradual increase of
the internal current-limit level at start-up for the purpose of
reducing input surge currents, and possibly for power-supply
sequencing. In Disable mode, the soft-start circuit holds the
Cton capacitor discharged to ground. When Enable goes high,
a 4µA current source charges the Cton capacitor up to 3.2V.
The resulting linear ramp causes the internal current-limit thresh-
old to increase proportionally from 20mV to 100mV. The out-
put capacitors charge up relatively slowly, depending on the
Cton capacitor value. The exact time of the output rise de-
pends on output capacitance and load current and is typically
1mS per nanofarad of soft-start capacitance. With no capaci-
tor connected, maximum current limit is reached typically within
10µS.


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