BD9532EKN_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]
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
The On Time(TON) at steady state is determined by
resistance value connected to FS pin.
But actually SW rising time and falling time come up
due to influence of the external MOSFET gate capacity
or switching speed and TON is increased.
The frequency is determined by the following formula
after TON, input current and the REF voltage are fixed.
F=
REF
VIN×TON
・・・(15)
Consequently, total frequency becomes lower than the
formula above.
TON is also influenced by Dead Time around the
output current 0A area in continuous mode.
This frequency becomes lower than setting frequency.
It is recommended to check the steady frequency in
large current area (at the point where the coil current
doesn’t back up).
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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