Operation – Ricoh RV5VH User Manual

Page 22

RV5VH1

××

/RV5VH2

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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

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

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.

SBD

OUT1

–

lim.

OSC

Vref

p_shift

VFM1

RV5VH1

××

Error Amp.1

SBD

OUT1

EXT1

–

OSC

Vref

p_shift

RV5VH2

××

Error Amp.1

VFM1

NPN Tr.