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Rev.1.2

■Functional description

ELM621LA is a step-up converter with the constant frequency and current-mode PWM. It provides the func-

tion to set the output voltage ranging from 2.7V to 30V by an external resistor divider. The output voltage (Vout)

can be obtained by the following formula:

Vout = 1.25 * ( 1+ R1 / R2 )

The duty cycle D of a step-up converter is defined as the following:

D = Ton * Fosc * 100% ≈ (1-Vin / Vout) * 100%

where Ton is the main switch on time, and Fosc is the oscillator frequency.

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. ELM621LA switches at a constant

frequency (1.3MHz) and regulates the output voltage. The PWM comparator modulates the power transferred to

the load by changing the inductor peak current based on the feedback error voltage during each cycle.The main

switch is turned on for a certain time to ramp the inductor current at each rising edge of the internal oscillator

under normal operation whereas off when the peak inductor current is above the error voltage.

2) Soft-start

ELM621LA features an internal soft-start function, which reduces inrush current and overshoot of the output

voltage. Soft-start can be achieved by ramping up the reference voltage (Vref), which is applied to the input of

the error amplifier.

3) Thermal shutdown

As soon as the junction temperature exceeds the typical 145°C, the device goes into thermal shutdown and the

main switch is latched off in this mode.

4) Undervoltage-lockout (UVLO)

The undervoltage-Iockout (UVLO) circuit prevents mal-operation of the device at low input voltage. It pre-

vents the converter from turning on the main switch under undefined conditions.

5) Shutdown

Driving the EN pin low can make the device enter the shutdown mode, during which the supply current drops

to 0.01µA. The capacitance and load at the output determine the rate at which Vout decays. The Vout is con-

nected to the input through the inductor and output rectifier, holding the output voltage to one diode drop below

Vin when the converter is shut down.

6) Output voltage

The output voltage is set using the FB pin and a resistor divider connected to the output as shown in standard

circuit. Schematic below. The output voltage (Vout) can be calculated according to the voltage of the FB pin (Vfb)

and ratio of the feedback resistors by the following equation, where (Vfb) is 1.25V:

Thus the output voltage is:

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