CMS06 datasheet(26/63 Pages) NISSHINBO

26 / 63 page

R1245x

No. EA-269-240903

VOLTAGE BETWEEN Lx PIN AND BST PIN

In the boot-strap style switching regulator, when the Lx pin voltage becomes lower than the regulator

which supplies BST voltage, CBST is charged.

By this charge, while the Lx pin voltage is "H", high side switch can be turned on continuously. Therefore,

if Lx pin voltage does not become lower than the BST voltage supply regulator, switching may be

abnormal. In the R1245, the output voltage of the BST voltage supply regulator is set at 5V. The abnormal

switching may be caused by the following conditions:

・

VOUT>5V, the difference between VIN and VOUT is small, inductor current is discontinuous by light

load

When the inductor current is continuous, or load current is big enough even if the discontinuous mode,

the forward current of the diode will make Lx pin voltage down and CBST is charged, but at light load, Lx

pin voltage does not become low enough against the BST voltage supply regulator output(5V). The

voltage of CBST is not high enough and drive capability will be down. (Figure 3-①) Due to the lack of the

drive capability, VOUT cannot be maintained, and under-shoot happens to VOUT, Lx pin voltage may become

lower than the BST voltage supply regulator output (5V), but the error amplifier operation may be

abnormal. When the charge of CBST is recovered and normal switching starts, VOUT becomes back to set

output voltage. However, after recovering the VOUT, to recover the error amplifier's operation, some

response time is necessary, during this response time, VOUT may be over-shoot. (Figure 3-②) As a result,

LX pin voltage cannot be low enough against the BST voltage supply regulator output voltage (5V), under-

shoot and over-shoot may be repeated. (Figure 4)

CMS06 Datasheet(PDF) 26 Page - Nisshinbo Micro Devices Inc.