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19
RV5VH1
××
/RV5VH2
××
C1
L
FB
V
OUT2
V
OUT1
EXT2
+
–
RV5VH1
××
/RV5VH2
××
Error Amp.2
VFM2
CSW
6
1
7
SBD
+
PMOS
OSC
C2
R2
R1
The DC/DC2 can operate by a voltage of “V
OUT1
”. A change in the V
OUT2
will feed back to the internal error amplifier
through external voltage setting resistors. The reference voltage should be provided from externally fixed power supply
such as V
OUT1
.
When the feed back voltage to the cmp2 is higher than the ground voltage the error amplifier enables oscillation or other-
wise will stop oscillation.
Pulses from the “OSC” circuit have a duty cycle of 50% and it makes VFM operation allowable. There might be certain
cases that the duty cycles becomes smaller temporarily at light load current. The output of “EXT2” is driven by CMOS
buffer operated V
OUT1
and GND.
A PMOS driver will be connected to the “EXT2” pin and its switching operation generates negative output voltage through
energy accumulated in an inductor.
The DC/DC1 can be shut down by CSW pin. When the CSW pin is “H”, V
DD
level, the DC/DC1 is enabled and when the
CSW pin is “L”, GND level, the DC/DC1 is disabled. The EXT2 pin outputs High while the DC/DC2 is disabled.
Set output voltage DC/DC Converter2
V
OUT2
is described as follows:
V
OUT1
:R1=|–V
OUT2
| : R2
|–V
OUT2
|=V
OUT1
×
R2/R1
/ The FB voltage is controlled to 0V and V
OUT1
is provided externally
thus, any output voltage of DC/DC2 can be set by changing R1 or/and R2.
Certain temperature coefficient of V
OUT2
can be set by using R1,R2 having such temperature characteristics.
DC/DC Converter 2