ELM600LA 800mA 1.4MHz
high efficiency synchronous PWM step down DC/DC converter
■Application notes
ELM600LA is constant frequency current mode PWM step down converter which is suitable for applications
that require high efficiency and small assembly area and use low voltage, such as Li-ion battery. By external
resistor divider, output voltage of ELM600LA can be set within the range from 0.6V to 6V. To provide opera-
tion with high efficiency, ELM600LA consists of main switch and synchronous rectifier while external Schottky
diode is not necessary. Duty cycle of ELM600LA can reach upto 100%. Duty cycle D of step down converter is
defined as follows:
D = Ton × Fosc × 100% ≈ (Vout/Vin) × 100%
Ton=main switch on time; Fosc=oscillator frequency (1.4MHz); Vout=output voltage;Vin=input voltage
1) Current mode PWM control
Slope compensated current mode control PWM provides superior load and line response, and protection of in-
ternal main switch and synchronous rectifier to current limit by stable switching and switch cycle. ELM600LA
switches at a constant frequency (1.4MHz) and regulates output voltage. During each cycle, the PWM compara-
tor modulates the power transferred to load by changing inductor peak current which is based on the feedback
error voltage. Under normal operation, the main switch is turned on for a certain time to ramp the inductor
current at each rising edge of the internal oscillator, and off when inductor peak current is above error voltage.
When the main switch is turned off, the synchronous rectifier will be turned on immediately and stay on until
either the next cycle begins or the inductor current drops to zero. With light load, ELM600LA would skip pulses
to improve efficiency.
2) Dropout operation
ELM600LA allows the main switch to remain on for more than one switching cycle and increases the duty
cycle while the input voltage is dropping under the output voltage. When duty cycle reaches 100%, the main
switch transistor would be on continuously to supply current to output until current limit generates. Output volt-
age then is input voltage minus voltage drop across main switch and inductor.
3) Short circuit protection
ELM600LA consists of short circuit protection; when output is short 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 normal values once the short circuit condition is recovered and the feedback voltage goes back to 0.6V.
4) Maximum load current
ELM600LA can operate when input voltage is down to 2.5V; however, the maximum load current would de-
scend because of the increase IR drop which results from the ON resistor of main switch and synchronous recti-
fier along with the drop of input voltage. The slope compensation signal would reduce the peak inductor current
to avoid sub-harmonic oscillations when duty cycles is larger than 50%; on the contrary, it would increase the
peak inductor current when duty cycle is decreased, as which is shown in the graph.
Rev.1.1
10 - 6
ELM-TECH [ ELM Technology Corporation ]
