L7250
4.4 Negative Voltage Regulator (CUK configuration)
Programming the Test Register Reg 0FH to ‘00101001’ it is possible to re-configurate the negative regulator
loop inverting its polarity. All the others test register (Reg0FH) configurations are resetting to the default nega-
tive voltage regulator loop polarity (take care to avoid the test register bits modification if the ‘CUK configuration’
hardware is present and the negative regulator is enabled).
The functionality of the regulator is the same descripted on the previous paragraph with the difference that the
loop polarity is reversed to permit to drive the external Nch component.
During this operation the nominal value of the triangular wave oscillator frequency is always fixed to 1 MHz.
The NVR is controlled via serial port (using the Reg. 05H -> b1 the regulator could be turned on and off).
Take care to program correctly the Test Register to enter the CUK configuration before to enable the NVR.
During the power-up and power-down phases the regulator is always off being the serial port in reset status then
the VnegEn bit equal to zero. During those phases the N_DRV output driver is in tri-state condition then an ex-
ternal pull-down to assure the Nch off condition must be considered.
Figure 18. Negative regulator (CUK configuration) - Test register => 00101001
V R E F 2 5
V C C 5
21
R 1ext
R 2ext
C 1ext
1Mhz
N _D R V
V b g
10
M 1ext
C fext
(typ 1.25 )
12
N _C O M P
5K
C cext
R cext
11
N _FE E D
40/46
STMICROELECTRONICS [ ST ]
