R16DS0137EU0100 Rev.1.0

Page 17

Apr 13, 2021

RAA210040 Datasheet

5.6

Undervoltage Lockout (UVLO)

When the input voltage is below the Undervoltage Lockout (UVLO) threshold, the module is disabled.

5.7

Soft Start-Up

The soft start-up reduces the inrush current during soft start-up. The soft-start block outputs a ramp reference to

the input of the error amplifier. This voltage ramp limits the inductor current and the output voltage rise time so that

the output voltage rises in a controlled fashion. When VFB is less than 0.1V at the beginning of the soft start-up,

the switching frequency is reduced to 200kHz, so that the output can start-up smoothly at light load conditions.

The RAA210040 supports pre-biased output condition during soft start-up. The default soft start-up period is

approximately 1ms.

5.8

External Synchronization Control

The operating frequency can be synchronized up to 4MHz by an external signal applied to the SYNC pin. The

rising edge of the SYNC signal triggers the rising edge of the PWM ON pulse. To ensure proper operation,

Renesas recommends using an external SYNC frequency within ±25% of the switching frequency set by the FS

pin.

5.9

Enable

The enable (EN) input allows you to control the turning on or off of the module for purposes such as power-up

sequencing. When the module is enabled, there is typically a 600µs delay for waking up the bandgap reference,

and then the soft start-up begins. EN should be held below the logic input low until VIN exceeds VUVLO rising

threshold.

5.10 Discharge Mode (Soft-Stop)

When a transition to shutdown mode occurs or the VIN UVLO is set, the discharge function is to ensure a defined

down-ramp of the output voltage and keep the output voltage close to 0V. The output voltage is discharged to

SGND through an internal 100Ω switch.

5.11 100% Duty Cycle

The RAA210040 features a 100% duty cycle operation to minimize the switching losses. When the input voltage

drops to a level that the RAA210040 can no longer maintain regulation at the output, the module completely turns

on the P-channel MOSFET. This is particularly useful in battery-powered applications to make full use of the

battery voltage and maximize the operation time.

Figure 35. External Frequency Synchronization Waveform, VIN = 5V, VOUT = 1.2V, fSYNC = 1.5MHz, IOUT = 0A

VSYNC 2V/Div

VOUT 10mV/Div

500ns/Div

1.5MHz