R5220x

9

TIMING CHART

1) IC start-up

The timing chart as shown in the next describes the operation starting the IC is enabled with CE. When the CE

pin voltage becomes higher than the threshold voltage, the IC’s operations starts. At first, only the voltage

regulator (VR) starts. The threshold level of the CE pin is between CE “H” input voltage and CE “L” input voltage.

output level becomes the set VR output voltage. At this moment, the output of Lx is “off”, (“Hi-Z”), the pin voltage,

Secondly, the Mode pin voltage is higher than the threshold voltage, internal operation of DC/DC starts. The

threshold level is between Mode “H” input voltage and Mode “L” input voltage. The soft-start circuit inside the

DC/DC converter’s operation is as follows:

(Case 1) DC/DC output voltage < VR output voltage

After the soft-start time, while the output voltage level is down from the VR output voltage to DC/DC output

voltage, the circuit is waiting for the start of DC/DC operation. When the output voltage reaches so set DC/DC

output voltage level, the actual DC/DC operation starts.

(Case 2) DC/DC output voltage> VR output voltage

The soft-start circuit of DC/DC converter makes the voltage reference unit of the IC rise gradually and be

constant. After the voltage reference unit reaches the constant level which the output voltage of DC/DC

converter can balance becomes the output voltage of VR, the set output voltage of DC/DC converter may be

realized.

Therefore, the soft-start time means the time range of starting to the time when the voltage reference unit

reaches the constant level, and the soft-start time is different from turning on speed in some cases. The

operation starting time depends on the ability of the power supply, the load current, the inductance value, the

capacitance value, and the voltage difference between the set VR output and the set DC/DC output.

If CE and Mode are on at once, the same operation as above is happened except the VR start-up and

Soft-start operation start at the same time.

If Mode signal is forced earlier than CE signal, this IC is stand-by until CE signal comes. Therefore when the

CE signal is set, the IC operation starts as above.

• V

OUT

voltage rising speed at start-up with power supply is affected by the next conditions:

• DC/DC operation starting time

the next formula.

*1

µA is the supply current of the IC itself for the output.

2.If the VR output < DC/DC output, the operation starting time is the soft-start time

+ starting operation time

which depends on the power supply, the load current, and the external components.