Datasheet

LXDC2UR series
Micro DC-DC converter
10
Dec 2014
PFM mode at light load PWM mode at heavy load
Nominal output voltage
7. Detailed Description
Mode selection
The MODE pin allows selecting the operating mode. If the MODE pin is pulled to logic low voltage (VMODL), the
converter operates automatic PFM and PWM mode. In this mode, the converter operates PFM mode at light load
current, and when the load current increase, the operating mode will change to PWM mode automatically. In this
mode, the converter can work in high efficiency over wide load current range. The transition current between PFM
and PWM is depend on Vin, Vout and other factors, but the ballpark threshold is about 70-150mA
If the MODE pin is pulled to logic high voltage (VMODH), the device operates in PWM forced mode. In this mode,
the converter operates in PWM mode even at light load current. The advantage of this mode is that the converter
operates with a fixed frequency that allows simple filtering of switching frequency. In this mode, the efficiency is
lower compared to the PFM mode at light load current.
UVLO (Under Voltage Lock Out)
The input voltage (Vin) must reach or exceed the UVLO voltage (2.05Vtyp) before the device begins the start up
sequence even when EN pin is kept high. UVLO function keeps away of an unstable operation at low Vin range
Soft Start
The device has an internal soft-start function that limits the inrush current during start-up. The soft-start system
progressively increases the switching on-time from a minimum pulse-width to that of normal operation. Because
of the function, the output voltage increases gradually from zero to nominal voltage at start-up event. The nominal
soft-start time is 120 μsec.
Enable
The device starts operation when EN is set high and starts up with soft start. For proper operation, the EN pin
must be terminated to logic high and must not left floating.
Pulling the EN pin to logic low forces the device shutdown. The shutdown quiescent current is 0.1 μA (typ).
Discharge Function
LXDC2UR series can actively discharge the internal output capacitor (Cout) when it turns off. The discharge time
depends on external output capacitance. LXDC2UR series do not need external capacitors, but if you use
external output capacitor its discharge time will become longer.
Over Current Protection
LXDC2UR series integrates a current limit function to protect internal components against heavy load or short
circuits. When over current event is removed, the device returns to the normal operation automatically.
Thermal Shutdown
As soon as the internal IC’s junction temperature exceeds 140
o
C (typ), the device goes into thermal shutdown.
The device returns to its normal operation when the Internal IC’s junction temperature again falls below
130
o
C(typ).