5

Rev.1.0

■Application notes

ELM601BA 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, wuch as Li-ion battery . By external

resistor divider, output voltage of ELM601BA can be set within the range from 1V to 5V. To provide operation

with high efficiency, ELM601BA consists of main switch and synchronous rectifier while external Schottky

diode is not necessary. Duty cycle of ELM601BA can reach upto 100%. Duty cycle of step down converter is

defined as follows:

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

Ton=main switch on time; Fosc=oscillator frequency (2.5MHz); Vout=output voltage; Vin=input voltage

1) Current mode PWM control

Slope compensated current mode PWM control 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. ELM601BA

switches at a constant frequency (2.5MHz) and regulates output voltage. During each cycle, the PWM compara-

tor modulates the power transferred to the load by changing inductor peak current which is based on the feed-

back 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 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.

2) Dropout operation

ELM601BA allows the main switch to remain on for more than one switching cycle and increases 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 voltage then

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

3) Short circuit protection

ELM601BA 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

ELM601BA can operatewhen 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 cycle 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.

7 -

a b c d e

SOT-25

a to e : Assembly lot No.

A to Z (I, O, X excepted) and 0 to 9

■Marking

high efficiency synchronous PWM step-down DC/DC converter