r2j20751np Renesas Electronics Corporation., r2j20751np Datasheet - Page 12

• Manufacturer Part Number: r2j20751np
• Description: Peak Current Mode Synchronous Buck Controller With Power Mos Fets
• Manufacturer: Renesas Electronics Corporation.

R2J20751NP
Design example;
1. Flat band gain of error amplifier
2. Selecting the Cf value to determine the frequency of the zero.
R07DS0240EJ0100 Rev.1.00
Jan 26, 2011
Specification: L = 470 nH, Co = 600 F, Fsw = 500 kHz, Vin = 5 V, Vout = 1.5 V, R1 = 1 k, R2 = 1 kΩ,
The flat band gain is; Af = Rf / (R1 // R2)  4 / 5  {R2 / (R1 + R2)}
Hence,
In the Bode plot, the total gain should be lower than 1 (0 dB) at the switching frequency.
The total gain at Fsw (= Asw) depends on the flat-band gain, so Af should be expressed as follows;
In the typical way, the value chosen for Asw is in the range from 0.1 to 0.5, since this produces a stable control loop.
The transient response will be faster if a large Asw is adopted, but the system might be unstable.
We choose 0.2 for Asw in the example below.
Therefore, we select a value of 24 k for Rf.
The frequency of the zero established by Cf and Rf is about ten times the frequency of the pole for the power stage
and modulator.
We must start with the dc gain of the power stage and modulator.
Here VCS0 is the peak ac voltage on CS pin when the load current is zero, thus
Rf = 5 / 4  Af  (R1 // R2) / {R2 / (R1 + R2)} ......(1)
Af = Asw  2   Fsw  Co  RCS / Nt ……(2)
Af = 0.2  2   500 kHz  600 F  820  / 13700 = 22.564
Rf = 5 / 4  22.564  0.6 k / (2 / 3) = 25.385 k
A0 =
VCS0 = 0.5  RCS  (Vin – Vout)  Vout / (L  Vin  Fsw) / 13700 ……(4)
Vout
R1
R2
Here, Nt = Idh / Ics = 13700
SQRT {Vin
= 0.5  820   (5 V – 1.5 V)  1.5 V / (470 nH  5 V  500 kHz) / 13700
= 0.134 V
RCS = 820 Ω
2
8
Nt/RCS
Rf
FB
L
Figure 3 Error Amplifier Compensation
Vin
REF
L
Cf
Fsw
Vin
(VCS0
Fsw
EO
Nt / RCS) }
10k
40k
Amp-out to current-sense comparator
(3)
Page 12 of 25
Preliminary