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http://www.elm-tech.com

Rev.1.0

ELM600xB is constant frequency current mode PWM step down converter. ELM600xB is optimized for low

voltage, Li-ion battery, powered applications where high efficiency and small size are critical. The device inte-

grates both a main switch and a synchronous rectifier, which provides high efficiency and eliminates an external

Schottky diode. ELM600xB can achieve 100% duty cycle. The duty cycle D of a step down converter is defined

as:

D = Ton × Fosc × 100% ≈ (Vout/Vin) × 100%

Where Ton is the main switch on time, Fosc is the oscillator frequency (1.5MHz), Vout is the output voltage

and Vin is the input voltage.

1) Current mode PWM control

Slope compensated current mode PWM control provides stable switching and cycle-by-cycle current limit for

superior load and line response and protection of the internal main switch and synchronous rectifier. ELM600xB

switches at a constant frequency (1.5MHz) and regulates the output voltage. During each cycle the PWM com-

parator modulates the power transferred to the load by changing the inductor peak current based on the feedback

error voltage. During normal operation, the main switch is turned on for a certain time to ramp the inductor cur-

rent at each rising edge of the internal oscillator, and switched off when the peak inductor current is above the

error voltage. When the main switch is off, the synchronous rectifier will be turned on immediately and stay on

until either the next cycle starts or the inductor current drops to zero. The device skips pulses to improve effi-

ciency at light load.

2) Dropout operation

ELM600xB allows the main switch to remain on for more than one switching cycle and increases the duty cy-

cle while the input voltage is dropping close to the output voltage. When the duty cycle reaches 100%, the main

switch is held on continuously to deliver current to the output up to the P MOSFET current limit. The output

voltage then is the input voltage minus the voltage drop across the main switch and the inductor.

3) Short circuit protection

ELM600xB has short circuit protection. When the output is shorted to ground, the oscillator frequency is re-

duced to prevent the inductor current from increasing beyond the P MOSFET current limit. The frequency will

return to the normal values once the short circuit condition is removed and the Vout reaches regulated voltage.

4) Maximum load current

ELM600xB can operate down to 2.5V input voltage; however the maximum load current decreases at lower

input due to large IR drop on the main switch and synchronous rectifier. The slope compensation signal reduces

the peak inductor current as a function of the duty cycle to prevent sub-harmonic oscillations at duty cycles

greater than 50%. Conversely the current limit increases as the duty cycle decreases.

■Application notes