RT9206GS Richtek USA Inc, RT9206GS Datasheet - Page 14

• Manufacturer Part Number: RT9206GS
• Description: IC CTRLR DUAL SYNC BUCK 16SOP
• Manufacturer: Richtek USA Inc

Specifications of RT9206GS

• Topology: Step-Down (Buck) Synchronous (1), Linear (LDO) (2)
• Function: Any Function
• Number Of Outputs: 3
• Frequency - Switching: 200kHz ~ 600kHz
• Voltage/current - Output 1: Controller
• Voltage/current - Output 2: Controller
• Voltage/current - Output 3: Controller
• W/led Driver: No
• W/supervisor: No
• W/sequencer: No
• Voltage - Supply: 4.75 V ~ 28 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

RT9206
The AC impedance of output capacitor at operating
frequency is quite smaller than the load impedance, so
the ripple current ( ΔI
through output capacitor. The output ripple voltage is
described as :
where ΔV
For electrolytic capacitor application, typically 90~95% of
the output voltage ripple is contributed by the ESR of output
capacitor. So Equation (4) could be simplified as :
Users
ESR.
www.richtek.com
14
ΔV
ΔV
ΔV
ΔV
Figure 2. The related waveforms of output capacitor.
OUT
OUT
OUT
OUT
could connect capacitors in parallel to get calculated
V
V
OC
OR
i
i
L
c
0
0
= ΔV
= ΔI
= ΔI
= ΔI
OR
d
dt
is caused by ESR and ΔV
iL
L
L
L
=
OR
×
×
×
V
IN
r +
r +
r
C
C
C
+ ΔV
-V
t1
L
OUT
L
Co
1 V
8 C L
) of the inductor current flows mainly
1
OC
∫
OUT
T
t1
O
t2
S
ic t
t2
(1- D)T
d
1/2
d
dt
ΔI
i
L
=
L
S
2
V
OC
OUT
L
I
by capacitance.
O
Δ
ΔI
I
L
Δ
x r
L
V
c
OC
(2)
(3)
(4)
(5)
Input Capacitor Selection
The selection of input capacitor is mainly based on its
maximum ripple current capability. The buck converter
draws pulsewise current from the input capacitor during
the on time of S1 as shown in Figure 1. The RMS value of
ripple current flowing through the input capacitor is
described as :
The input capacitor must be cable of handling this ripple
current. Sometime, for higher efficiency the low ESR
capacitor is necessarily.
Power MOSFET Selection
The selection of MOSFETs is based on consideration of
maximum gate-source voltage (Vgs), drain-source voltage
(Vdss), maximum drain current (Id), drain-source on-state
resistance R
MOSFETs are driven by V
as 6.0V. Low threshold voltage MOSFET should be
selected to guarantee that it could fully turn on at
Vgs = 6.0V.
The total power dissipation of external MOSFETs consists
of conduction and switching losses. The conduction losses
of high side and low side MOSFETs are described by
equation (7) and (8), respectively.
(High-side MOSFET)
(Low-side MOSFET)
Where
The total switching loss is approximated as.
Where
θ
V
time.
I
t
f
I
P
P
P
r
OUT
s
DS(OFF)
rms
r
L-CON
H-CON
SW
and t
= Switching frequency
is temperature dependency of R
= I
= Load current
= I
O
f
is voltage from drain to source at MOSFET off
are rise-time and fall-time, respectively.
= I
OUT
= I
D(1- D)
0
2
2
0
×
DS(ON)
× ×
×
(1-D) R
D R
V
DS(OFF)
2
(A)
DS(ON)
×
and thermal management. The
DS(ON)
×
INT
(tr tf) fs
×
θ
that is internally regulated
+
r
×
θ
(W)
r
DS9206-12 April 2008
×
(W)
ds(on)
(W)
(6)
(8)
(7)
(9)