CMS06 datasheet(29/63 Pages) NISSHINBO

29 / 63 page

R1245x

No. EA-269-240903

MINIMUM ON TIME

The minimum On time of the R1245 is Typ.110ns. 110ns derives from the current sense circuit delay

time and stability.

The R1245 has adopted current mode control without an external sense resistance. Instead of the

external sense resistance, the on resistance of the N-channel driver: RON is used. VIN-VLX=ILX×RON,

therefore ILX (inductor current) can be sensed. ILX can be sensed during the on time of the N-channel

driver, (LX="H"), if Lx switching surge is sensed just after turning on, the operation may be abnormal,

therefore, just after turning on the N-channel driver, sensing is stopped for a short period and avoid the

error by the switching surge.

Therefore, during this sensing delay time, current mode control and current limit cannot operate

properly.

Figure 5. X-axis: On time, Y-axis: Current limit

By this current sense delay time, 110ns or less, current limit circuit has also delay time, and detecting

current increases drastically. This delay time includes the signal delay time from the current sense circuit

to the driver.

Figure 5. On time and Lx current limit, Lx pin current peak value ILXLIM

This could be applied to the current mode control. Therefore, 110ns or less, current mode control

does not work correctly, and operation becomes like a low stability voltage control mode.

Thus, if the condition of switching on time of the R1245 becomes less than 110ns, stability and

current limit detecting accuracy are deteriorated. The higher the switching frequency and the input

voltage, the higher current limit detecting threshold is and output and GND short must be cared. Under

such conditions, the stability must be assured by the external components, and over current protection

other than the IC is necessary. (ex. Using a fuse with the IC.)

1.8

2.2

2.4

2.6

2.8

100

150

200

250

300

350

Ontime [ns]

CMS06 Datasheet(PDF) 29 Page - Nisshinbo Micro Devices Inc.