r1230d RICOH Co.,Ltd., r1230d Datasheet - Page 10

• Manufacturer Part Number: r1230d
• Description: Pwm/vfm Step-down Dc/dc Converter With Synchronous Rectifier
• Manufacturer: RICOH Co.,Ltd.

respectively described as Ronp and Ronn, and the DC resistor of the inductor is described as R
Consider ILmax, condition of input and output and select external components.
As for Adjustable Output type, feedback pin (V
Output Voltage, V
Thus, with changing the value of R1 and R2, output voltage can be set in the specified range.
In the DC/DC converter, with the load current and external components such as L and C, phase might be behind 180
degree. In this case, the phase margin of the system will be less and stability will be worse. To prevent this, phase
margin should be secured with proceeding the phase. A zero is formed with R1, R2, and Cb.
Fpole 1/2
Considering external components, LC, set the appropriate value of R1+R2 and Cb.
Rb is effective for reducing the noise on V
When the R1 30k , additional Rb is not necessary, so it should be wired to V
1. Inductor
2. Capacitor
Rev. 1.12
(Wherein, Ripple Current P-P value is described as I
When P-channel Tr. of Lx is “OFF”(N-channel Tr. is “ON”):
Put Equation 4 to Equation 3 and solve for ON duty of P-channel transistor, ton/(toff+ton)=D
Ripple Current is as follows;
wherein, peak current that flows through L, and Lx Tr. is as follows;
When P-channel Tr. of Lx is ON:
Fzero 1/(2
The above explanation is directed to the calculation in an ideal case in continuous mode.
When ton<tonc, the mode is the discontinuous mode, and when ton=tonc, the mode is the continuous mode.
ILmax=I
Select an inductor that peak current does not exceed ILmax. If larger current than allowable current flows, magnetic
saturation occurs and make transform efficiency worse.
Supposed that the load current is at the same, the smaller value of L is used, the larger the ripple current is.
Provided that the allowable current is large in that case and DC current is small, therefore, for large output current,
efficiency is better than using an inductor with a large value of L and vice versa.
OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS
tonc =T V
V
L I
D
I
How to Adjust Output Voltage and about Phase Compensation
External Components
RP
IN
ON
RP
=(V
=V
=(V
/toff = R
V
OUT
V
IN
OUT
OUT
OUT
OUT
-V
LC
+(Ronp+R
IN
OUT
+I
=V
(R1+R2) Cb)
: R1+R2=VFB: R2
-Ronn I
OUT
/V
RP
L
FB
OUT
OUT
-Ronp I
/2
I
OUT
(R1+R2)/R2
is as following equation;
OUT+
L
Equation 2
+ V
) I
OUT
OUT
OUT
R
L
-R
+L I
I
+ Ronn I
L
OUT
I
OUT
RP
)/(V
/ton
) D
IN
OUT
+ Ronn I
FB
ON
, therefore, add Rb so that R1+Rb=30k .
/fosc/L
FB
) voltage is controlled to maintain 0.8V.
OUT
RP
- 10 -
-Ronp I
, ON resistance of P-channel Tr. and N-channel Tr. of Lx are
OUT
) Equation 5
Equation 3
Equation 4
Equation 6
Equation 7
FB
directly.
ON
,
L
.)