Datasheet

LM3526

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SNVS054E –FEBRUARY 2000–REVISED MARCH 2013

A 0.01 µF ceramic capacitor is recommended on each port directly between the V

bus

and ground pins to prevent

EMI damage to other components during the hot-detachment.

Adequate capacitance must be connected to the input of the device to limit the input voltage drop during a hot-

plug event to less than 330 mV. For a few tens of µs, the host must supply the in-rush current to the peripheral,

charging its bulk capacitance to V

bus

. This current is initially supplied by the input capacitor. A 33 µF 16V

tantalum capacitor is recommended.

In choosing the capacitors, special attention must be paid to the Effective Series Resistance, ESR, of the

capacitors to minimize the IR drop across the capacitor's ESR.

SOFT START

To eliminate the upstream voltage droop caused by the high in-rush current drawn by the output capacitors, the

maximum in-rush current is internally limited to 1.5A.

TRANSIENT OVER-CURRENT DELAY

High transient current is also generated when the switch is enabled and large values of capacitance at the output

have to be rapidly charged. The in-rush currents created could exceed the short circuit current limit threshold of

the device forcing it into the current limit mode. The capacitor is charged with the maximum available short circuit

current set by the LM3526. The duration of the in-rush current depends on the size of the output capacitance and

load current. Since this is not a valid fault condition, the LM3526 delays the generation of the fault flag for 1 ms.

If the condition persists due to other causes such as a short, a fault flag is generated after a 1 ms delay has

elapsed.

The LM3526's 1 ms delay in issuing the fault flag is adequate for most applications. If longer delays are required,

an RC filter as shown in Figure 21 may be used.

Figure 21.

PCB LAYOUT CONSIDERATIONS

In order to meet the USB requirements for voltage drop, droop and EMI, each component used in this circuit

must be evaluated for its contribution to the circuit performance. The PCB layout rules and guidelines must be

followed.

• Place the switch as close to the USB connector as possible. Keep all V

bus

traces as short as possible and use

at least 50-mil, 1 ounce copper for all V

bus

traces. Solder plating the traces will reduce the trace resistance.

• Avoid vias as much as possible. If vias are used, use multiple vias in parallel and/or make them as large as

possible.

• Place the output capacitor and ferrite beads as close to the USB connector as possible.

• If ferrite beads are used, use wires with minimum resistance and large solder pads to minimize connection

resistance.

Product Folder Links: LM3526