ELM605DA
2A synchronous step-down DC/DC converter
■Functional description
ELM605DA is high-efficiency DC-to-DC step-down converter and capable of delivering up to 2A of output current. It
operates in pulse-width modulation at 1MHz fixed frequency with 2.5V to 6V input voltage and provides output volt-
age ranging from 0.8V to VIN. The high switching frequency allows for the use of smaller external components, and
internal synchronous rectifiers improve efficiency and eliminate external Schottky diode. Using the on-resistance of the
internal high-side MOSFET to sense switching currents eliminates current-sense resistors, further enhancing the effi-
ciency and reducing the cost.
1. Current mode PWM control
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. ELM605DA switches at 1MHz fixed
frequency and regulates the output voltage. The main switch is turned on for a certain period to ramp the inductor cur-
rent at each rising edge of the internal oscillator under normal operation whereas 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 the next cycle starts.
2. Dropout operation
ELM605DA allows the main switch to remain on for more than one switching cycle and increases the duty
cycle while the input voltage decreases close to the output voltage. When the duty cycle reaches 100%, the main
switch still keeps on in order to deliver the 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
ELM605DA features short circuit protection. When the output is shorted to ground, the oscillator’s frequency
is reduced to prevent the inductor’s current from increasing beyond the P-Channel MOSFET current limit. The
P-Channel MOSFET current is reduced to lower the short circuit current. The frequency and current limit will
return to the normal values once the short circuit condition is removed and the feedback voltage restores above
0.3V.
4. Internal soft-start
ELM605DA supports an internal soft-start function, which reduces inrush current and overshoot of the output
voltage. Soft-start is achieved by ramping up the reference voltage (Vref), which is applied to the input of the
error amplifier. The typical soft-start time is about 1 ms, and it depends on the component’s values on AP circuit.
5. Thermal Shutdown
As soon as the junction temperature exceeds the typical 150°C, the device goes into thermal shutdown. In this
mode, the P-Channel switch and N-Channel MOSFETs are latched off.
6. Under-voltage lockout
The under-voltage lockout circuit prevents mal-operation of the device at low input voltage. It prevents the con-
verter from turning on the switch or MOSFET under undefined conditions.
7. Enable
Connect EN to ground forces the device into shutdown mode, whereas to VIN or floating enables the device
.Pulling the EN low forces the IC to enter the shutdown mode, in which the P-Channel MOSFET and N-Channel
MOSFETs are turned off and the whole device is shut down. If an output voltage is present during shut down,
this could be an external voltage source or super cap. The reverse leakage current is specified under electrical
parameter table. On the contrary, pulling the EN high starts up the ELM605DA in the way as described soft-start
section.
Rev.1.2
13 - 4
ELM-TECH [ ELM Technology Corporation ]
