RT6908
Boost Diode Selection
The schottky diode is a good choice for any asynchronous
boost converter due to the small forward voltage. However,
when selecting a schottky diode, important parameters
such as power dissipation, reverse voltage rating, and
pulsating peak current must all be taken into
consideration. Asuitable schottky diode's reverse voltage
rating must be greater than the maximum output voltage,
and its average current rating must exceed the average
output current.
ΔI
L
Input Current
Inductor Current
Output Current
Time
(1-D)T
S
Output Ripple
Voltage (ac)
Boost Input Capacitor Selection
Time
Low ESR ceramic capacitors are recommended for input
capacitor applications. Low ESR will effectively reduce
the input ripple voltage caused by the switching operation.
Another consideration is the voltage rating of the input
capacitor, which must be greater than the maximum input
voltage.
ΔV
OUT1
Figure 4. The Output Ripple Voltage without the
Contribution of ESR
Boost Loop Compensation
The voltage feedback loop can be compensated with an
external compensation network consisted of R22 and C30.
Choose R22 to set the high frequency integrator gain for
fast transient response. And choose C30 to set the
integrator zero to maintain stability.
Boost Output Capacitor Selection
Output ripple voltage is an important index for estimating
the performance. A120μF low ESR OS-CAP is sufficient
for most applications. This portion consists of two parts,
one is the product of IIN and ESR of output capacitor, another
part is formed by charging and discharging process of
output capacitor. As shown in Figure 4, ΔVOUT1 can be
evaluated based on the ideal energy equalization.
According to the definition of Q, the Q value can be
calculated as the following equation :
VI/O Synchronous Buck Converter
The buck converter is a high efficiency PWM architecture
with 500kHz operation frequency and fast transient
response. The converter drives an internal N-MOSFET,
connected between the VINB1 and LXB1 pin. Connect a
100nF low ESR ceramic capacitor between the BOOT1
pin and LXB1 pin to provide gate driver voltage for the high
side MOSFET.
⎡
⎣
⎤
1
2
1
2
1
2
⎛
⎜
⎝
⎞ ⎛
+ I
⎞
⎟
⎠
Q =
×
I + ΔI −I
−
ΔI −I
L OUT
IN
L
OUT
IN
⎟ ⎜
⎠ ⎝
⎢
⎥
⎦
V
1
OSC
IN
×
×
=C ×ΔV
OUT OUT1
V
OUT
f
Where fOSC is the switching frequency and the ΔIL is the
inductor ripple current. Move COUT to the left side to
estimate the value of ΔVOUT1 as the following equation :
VI/O Buck Output Voltage Setting
The regulated default output voltage is as shown in the
following equation :
R2
R3
D×I
⎛
⎝
⎞
⎟
⎠
OUT
×f
V
I/O
= VFBB1× 1 +
, where VFBB1 = 0.8V (typ.)
ΔV
=
⎜
OUT1
η ×C
OUT OSC
Finally, the output ripple voltage can be determined as
following equation :
The recommended value for R2 should be up to 10kΩ
without some sacrificing. To place the resistor divider as
close as possible to the chip can reduce noise sensitivity.
The output voltage also can be adjusted from −7% to 7%
by setting the I2C register 03h [3:0].
D×I
OUT
×f
ΔV
= I ×ESR+
IN
OUT
η ×C
OUT OSC
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6908-01 March 2013
www.richtek.com
17
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