Datasheet

PWM

VCC

COMP

VOUT

LM5002

SHUTDOWN

and STANDBY

SOFT-START

CAPACITOR

1.3V

1.26V

LM5002

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SNVS496D –JANUARY 2007–REVISED MARCH 2013

SOFTSTART

Soft-start (SS) can be implemented with an external capacitor connected to COMP through a diode as shown in

Figure 20. The COMP discharge MOSFET conducts during Shutdown and Standby modes to keep the COMP

voltage below the PWM offset (1.3V), which inhibits PWM pulses. The error amplifier will attempt to raise the

COMP voltage after the EN pin exceeds the 1.26V standby threshold. Because the error amplifier output can

only sink current, the internal COMP pull-up resistor (~5 kΩ) will supply the charging current to the SS capacitor.

The SS capacitor will cause the COMP voltage to gradually increase, until the output voltage achieves regulation

and FB assumes control of the COMP and the PWM duty cycle. The SS capacitor continues charging through a

large resistance, R

, preventing the SS circuit from interfering with the normal error amplifier function. During

shutdown, the VCC diode discharges the SS capacitor.

Figure 20. Soft-Start

Printed Circuit Board Layout

The LM5002 Current Sense and PWM comparators are very fast and may respond to short duration noise

pulses. The components at the SW, COMP, EN and the RT pins should be as physically close as possible to the

IC, thereby minimizing noise pickup on the PC board tracks.

The SW output pin of the LM5002 should have a short, wide conductor to the power path inductors, transformers

and capacitors in order to minimize parasitic inductance that reduces efficiency and increases conducted and

radiated noise. Ceramic decoupling capacitors are recommended between the VIN pin to the GND pin and

between the VCC pin to the GND pin. Use short, direct connections to avoid clock jitter due to ground voltage

differentials. Small package surface mount X7R or X5R capacitors are preferred for high frequency performance

and limited variation over temperature and applied voltage.

If an application using the LM5002 produces high junction temperatures during normal operation, multiple vias

from the GND pin to a PC board ground plane will help conduct heat away from the IC. Judicious positioning of

the PC board within the end product, along with use of any available air flow will help reduce the junction

temperatures. If using forced air cooling, avoid placing the LM5002 in the airflow shadow of large components,

such as input capacitors, inductors or transformers.

Product Folder Links: LM5002