BD95513MUV_10 Datasheet, PDF (13/18 Pages) Rohm – Switching Regulator with MOS FET for DDR-SDRAM Cores

BD95513MUV
Technical Note
4. Frequency Adjustment
500
450
400
350
300
250
200
150
100
50
0
50
From top: VIN=
5V
7V
12V
19V
25V
100
150
200
250
300
RFS[kΩ]
The resistance connected to the FS terminal adjusts the on-time
(t_ON) during normal operation as illustrated to the left. When t_ON,
input voltage and V_REF voltage are known, the switching frequency
can be determined by the following formula:
F=
VREF
VIN×tON
・・・(10)
However, real-life considerations (such as external MOSFET gate
capacitance and switching time) must be factored in as they affect
the overall switching rise and fall time. This leads to an increase
in t_ON, lowering the total frequency slightly.
Additionally, when output current lingers around 0A in continuous
mode, this “dead time” also has an effect upon t_ON, further lowering
the switching frequency. Confirm the switching frequency by
measuring the current through the coil (at the point where current
does not flow backwards) during normal operation.
The BD95513MUV operates by feeding the output voltage back through a resistive voltage divider.
set by the following equation (see schematic below):
R1+R2
1
Output Voltage =
R2
× VREF (0.7V) + ₂ ×ΔIL×ESR・・・(11)
The switching frequency is also amplified by the same resistive voltage divider network:
R1+R2
f_sw =
R2
×(frequency set by R_FS) [Hz]・・・(12)
The output voltage is
REF(0.7V)
FB
VIN
H³Reg^TM
CONTROLLA
RQ
S
SLLM^TM
SLLM
Driver
Circuit
VIN
Output Voltage
ESR
R1
R2
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© 2010 ROHM Co., Ltd. All rights reserved.
13/17
2010.10- Rev.A