Preliminary Datasheet

LP3315

LP3315– 00 Version1.0 Datasheet

Feb.-2012

www.lowpowersemi.com

Page 6 of 8

Applications Information

Inductor Selection

The recommended value of inductor for 30V applications are 4.7 to 22µH. Small size and better efficiency are the

major concerns for portable device, such as LP3315 used for mobile phone. The inductor should have low core

loss at 1.2MHz and low DCR for better efficiency. To avoid inductor saturation current rating should be considered.

Constant Output Voltage Control

The output voltage of the LP3315 can be adjusted by the divider circuit on the FB pin. Typical FB voltage is

1250mV. The output voltage can be calculated by the following Equations.

Power Sequence

In order to assure the normal soft start function for suppressing the inrush current the input voltage should be

ready before EN pulls high.

Soft-Start

The function of soft-start is made for suppressing the inrush current to an acceptable value at the beginning of

power on. The LP3315 provides a built-in soft-start function by clamping the output voltage of error amplifier so

that the duty cycle of the PWM will be increased gradually in the soft-start period.

Current Limiting

The current flow through inductor as charging period is detected by a current sensing circuit. As the value comes

across the current limiting threshold, the N-MOSFET will be turned off so that the inductor will be forced to leave

charging stage and enter discharging stage. Therefore, the inductor current will not increase over the current

limiting threshold.

OVP/UVLO/OTP

The Over Voltage Protection is detected by a junction breakdown detecting circuit. Once VOUT goes over the

detecting voltage, LX pin stops switching and the power N-MOSFET will be turned off. Then, the VOUT will be

clamped to be near VOVP. As the output voltage is higher than a specified value or input voltage is lower than a

specified value, the chip will enter protection mode to

prevent abnormal function. As the die temperature is higher then 160°C, the chip also will enter protection mode.

The power MOSFET will be turned off during protection mode to prevent abnormal operation.

Thermal Considerations

For continuous operation, do not exceed absolute maximum operation junction temperature. The maximum power

dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and

temperature difference between junction to ambient. The maximum power dissipation can be calculated by

following formula :

Where TJ(MAX) is the maximum operation junction temperature, TA is the ambient temperature and the qJA is the