rt9206 Richtek Technology Corporation, rt9206 Datasheet - Page 15

• Manufacturer Part Number: rt9206
• Description: High Efficiency, Synchronous Buck With Dual Linear Controllers
• Manufacturer: Richtek Technology Corporation

The MOSFET should be capable of handling the power
loss over the entire operating range.
Design Example:
Design the power stage for a synchronous step-down
converter having the following specifications:
V
frequency = 200kHz, to determine the value of inductor
and output capacitor (Using electrolytic capacitor).
First, select ripple current of inductor is 20% of output
current, from equation (1)
Select L = 15 H
From equation (5)
Select two electrolytic capacitors C = 470 F,r
in parallel.
Setting the Current Limit
The RT9206 limits output current by sensing low side
MOSFET voltage drop (V
voltage caused by on-state resistance R
as :
When V
activated and latch the controller. So the current limit
function can be set by MOSFETs selection. The relation
of maximum inductor current I
of MOSFET (R
Setting the Output voltage
The output voltage is set by external voltage divider and
reference voltage. The feedback pin (FB, LFB1, and LFB2)
is connected to the inverting input of error amplifier and is
referenced to 0.8V reference voltage at non-inverting input
as shown in Figure 3.The output voltage is set by the
following equation.
DS9206-11 March 2007
IN
= 12V, V
L = (12 - 5)
SD
V
>300mV, the current limit function will be
OUT
OUT
DS(ON)
= 5V, I
12 200K 0.2 5
= (1+
R
DS(ON)
V
) is described as :
OUT
SD
Ra
Rb
=R
5
=
= 5A, V
25mV=1 x r
) 0.8
SD
300 10
DS(ON)
) when it turns on. The drop
L(LIM)
I
L(LIM)
and on-state resistance
x I
OUT
= 14.58 H
-3
L
< 25mV, switching
DS(ON)
C
( )
is described
C
= 43m
(10)
(11)
(12)
If high value resistors are used, the input bias current of
FB pin could cause a slight increase in output voltage.
The output voltage set point can be more accurate by using
precision resistor.
Soft-start setting
Figure 4 shows the typical soft-start timing waveforms of
RT9206. The soft-start time of Buck converter can be set
by selecting the soft-start capacitance value. The delay
time between input voltage applied and output voltage
starting to ramp up (T
time from input voltage applied to output voltage buildup
(T
The effective soft-start time (T
ramps up from zero to set voltage is calculated as :
Besides, appropriate soft-start capacitor should be selected
so that the start-up current will not trigger the current limit
function. And make sure that the input power source could
supply the soft-start current.
The total time from input voltage applied to power good
signal pull-high (T
VR
Figure 3. The connected diagram of external voltage
) is calculated as :
(LFB1,LFB2)
T
SS
T
FB
VR
= (320
divider and reference voltage
T
= 57 C
PG
PGOOD
= 640 C
Rb
V
OUT
V
DELAY
V
SS
OUT
) is calculated as :
Ra
IN
10
) 10
) is calculated as: The total
SS
SS
6
) during that output voltage
0.8V
-
+
6
10
6
C
SS
RT9206
www.richtek.com
(ms) (15)
(ms) (14)
(ms) (13)
15