BD9533EKN_08 Datasheet, PDF (13/21 Pages) Rohm – Switching Regulators for DDR-SDRAM Cores

6.Setting frequency
3000
2500
2000
1500
1000
500
0
0
VIN=5V
7V
12V
16V
19V
REF=1.8V
50
100
150
200
RFS [kΩ]
Calculated frequency
using TON above
1200
1000
800
600
400
200
0
0
VIN=5V
7V
12V
16V
19V
50
100
150
200
Resistance [kΩ]
7. Setting standard voltage (REF)
VIN
REF
Outside
voltage
VOUT
H³Reg^TM
CONTROLLA
RQ
S
On Time (TON) at steady state is determined by
resistance value connected to FS pin as shown in the
figure. The frequency can be calculated with TON,
input voltage VIN, the reference voltage REF as
follows.
F=
REF
VIN×TON
・・・(15)
However, the actual frequency becomes lower than
the formula due to expanded TON time caused by the
rise/fall time of the MOSFET (This rise/fall time is
affected by the switching speed). TON is also
influenced by Dead Time under light load in
continuous mode. In this case the frequency
becomes lower than setting frequency.
The setting frequency noted above is under heavy
load (when the inductor current does not go back to
VIN through the body diodes of the external
MOSFET’s).
It is available to synchronize setting the reference
voltage (REF) with outside supply voltage [V] by using
outside power supply voltage.
VREG
R1
REF
R2
VOUT
VIN
H³Reg^TM
CONTROLLA
RQ
S
It is available to set the reference voltage (REF) by the
resistance division value from VREG in case it is not
set REF from an external power supply.
R2
REF=
×VREG [V]・・・(16)
R1+R2
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