LM5574MT/NOPB National Semiconductor, LM5574MT/NOPB Datasheet - Page 17

• Manufacturer Part Number: LM5574MT/NOPB
• Description: IC BUCK SYNC ADJ .5A 16TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®, SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM5574MT/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 70 V
• Current - Output: 500mA
• Frequency - Switching: 200kHz, 485kHz
• Voltage - Input: 6 ~ 75 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP
• Power - Output: 700mW
• Current, Input Bias: 3.7 mA
• Current, Input Offset: 50 μA
• Current, Output: 0.5 A
• Frequency, Oscillator: 200/485 kHz
• Package Type: TSSOP
• Regulator Type: Buck (Step-Down), Switching
• Resistance, Thermal, Junction To Case: 30 °C/W
• Temperature, Operating, Range: -40 to +125 °C
• Voltage, Input: 6 to 75 V
• Voltage, Offset: 0.7 V (COMP to PWM Comparator)
• Voltage, Output: 7.15 V
• Dc To Dc Converter Type: Step Down
• Pin Count: 16
• Input Voltage: 75V
• Output Voltage: 1.225 to 70V
• Switching Freq: 50 TO 500KHz
• Output Current: 500mA
• Efficiency: 85%
• Output Type: Adjustable
• Switching Regulator: Yes
• Mounting: Surface Mount
• Input Voltage (min): 6V
• Operating Temp Range: -40C to 125C
• Operating Temperature Classification: Automotive
• For Use With: LM5574BLDT - WEBENCH BUILD IT BOARD LM5574LM5574EVAL - BOARD EVALUATION FOR LM5574
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM5574MT; *LM5574MT/NOPB; LM5574MT

PCB LAYOUT AND THERMAL CONSIDERATIONS
The circuit in Figure 1 serves as both a block diagram of the
LM5574 and a typical application board schematic for the
LM5574. In a buck regulator there are two loops where cur-
rents are switched very fast. The first loop starts from the input
capacitors, to the regulator VIN pin, to the regulator SW pin,
to the inductor then out to the load. The second loop starts
from the output capacitor ground, to the regulator PGND pins,
to the regulator IS pins, to the diode anode, to the inductor
and then out to the load. Minimizing the loop area of these
two loops reduces the stray inductance and minimizes noise
and possible erratic operation. A ground plane in the PC
board is recommended as a means to connect the input filter
capacitors to the output filter capacitors and the PGND pins
of the regulator. Connect all of the low power ground connec-
tions (C
Connect the AGND and PGND pins together through the top-
side copper trace. Place several vias in this trace to the
ground plane.
The two highest power dissipating components are the re-
circulating diode and the LM5574 regulator IC. The easiest
method to determine the power dissipated within the LM5574
is to measure the total conversion losses (Pin – Pout) then
subtract the power losses in the Schottky diode, output in-
ductor and snubber resistor. An approximation for the Schot-
tky diode loss is P = (1-D) x Iout x Vfwd. An approximation for
the output inductor power is P = I
SS
, R
T
, C
RAMP
) directly to the regulator AGND pin.
OUT
2
x R x 1.1, where R is
17
the DC resistance of the inductor and the 1.1 factor is an ap-
proximation for the AC losses. If a snubber is used, an ap-
proximation for the damping resistor power dissipation is P =
Vin
and Csnub is the snubber capacitor.
The most significant variables that affect the power dissipated
by the LM5574 are the output current, input voltage and op-
erating frequency. The power dissipated while operating near
the maximum output current and maximum input volatge can
be appreciable. The operating frequency of the LM5574 eval-
uation board has been designed for 300kHz. When operating
at 0.5A output current with a 70V input the power dissipation
of the LM5574 regulator is approximately 0.6W.
The junction-to-ambient thermal resistance of the LM5574 will
vary with the application. The most significant variables are
the area of copper in the PC board, and the amount of forced
air cooling provided. The junction-to-ambient thermal resis-
tance of the LM5574 mounted in the evaluation board varies
from 90°C/W with no airflow to 60°C/W with 900 LFM (Linear
Feet per Minute). With a 25°C ambient temperature and no
airflow, the predicted junction temperature for the LM5574 will
be 25 + ((90 x 0.6) = 79°C. If the evaluation board is operated
at 0.5A output current, 70V input voltage and high ambient
temperature for a prolonged period of time the thermal shut-
down protection within the IC may activate. The IC will turn
off allowing the junction to cool, followed by restart with the
soft-start capacitor reset to zero.
2
x Fsw x Csnub, where Fsw is the switching frequency
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