Ricoh RV5VH - Manual (Page 22)

RV5VH1
××
/RV5VH2
××
18
OPERATION
• DC/DC Converter 1
The DC/DC1 uses input voltage as an initial power supply, once boost operation is started the boost output will be used for
the power supply of device itself. A change in the V OUT1 will feed back to the internal error amplifier through external volt-
age setting resistors and internal feed back resistors. When the feed back voltage is lower than the reference voltage the
error amplifier enables oscllation or otherwise will stop oscillation. The internal feed back resistor “R” which is fixed and
adjusted by laser trim can make the feed back input voltage to “Error Amp.1” stable. Pulses from the “OSC” circuit have a
duty cycle of 50% and it becomes 65 to 75%(at high side) through the “P_shift” circuit. The duty cycle may be smaller in a
short period of time with light load.
These clook pulses control VFM circuit and make it possible to operate as a boost converter. The output of L X1 is Nch open
drain, while the output of “EXT1” is driven by CMOS buffer and an external NMOS driver is also available instead of an
NPN transistor, in such cases the Rb and Cb are not necessary, a recommended Rb is 300
Ω.
When you use a MOSFET
for the EXT1, the input voltage should be high enough and you can get high effiiciency applications.
A current limit is available only for the RV5VH1 series, to prevent an excess current from flowing through Nch driver tran-
sistor.
The DC/DC1 can be shut down by CSW pin. When the CSW pin is High, V DD level, the DC/DC1 is enabled and when the
CSW pin is Low, GND level, the DC/DC1 is disabled. The EXT1 pin outputs “L” while the DC/DC1 is disabled.
C
L
SBD
V OUT1
V OUT1
L X1
R –
+
V LX lim.
OSC
Vref
p_shift
VFM1
V IN
RV5VH1
××
Error Amp.1
3
4
C
L1
SBD
V OUT1
V OUT1
EXT1
R –
+
OSC
Vref
p_shift
V IN
RV5VH2
××
Error Amp.1
VFM1
NPN Tr.
Cb
3
4
Rb