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SP6121 Datasheet(PDF) 35 Page - Sipex Corporation

Part # SP6121
Description  Low Voltage, Synchronous Step Down PWM Controller
PDF  42 Pages
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Manufacturer  SIPEX [Sipex Corporation]
Direct Link  http://www.sipex.com
Logo SIPEX - Sipex Corporation

SP6121 Datasheet(HTML) 35 Page - Sipex Corporation

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Rev 7/25/01
3
FIGURE 3. SP6121 Demo Board Schematic.
Demo Board Component Selection
The input capacitor, C6, can be either a ceramic or tantalum capacitor. Its choice
depends on the user’s system input voltage transient. If the highest possible efficiency
is required, a ceramic 47uF capacitor is recommended.
The demo board circuit includes a Schottky Diode, D1, across the low side switch. It
prevents Q2’s internal body diode from turning on and dissipating power during the
nonoverlap time when both Q1 and Q2 are off. At a 500kHz switching frequency, these
losses would decrease overall efficiency by 0.5 – 0.6%.
The output voltage is set by the resistor divider: R3/R4. The output voltage is calculated
using the following:
REF
OUT
V
R
R
V
÷
ø
ö
ç
è
æ
+
=
1
4
3
,
where VREF = 1.25V.
For the SP6121 demo board where VOUT = 2.5V. Choosing R4 equal to 10K, then
K
R
R
10
4
3
=
=
The considerations, tradeoffs and calculations required to select the power MOSFETs
(Q1, Q2), the inductor (L1), the input and output capacitors (C1, and C6, C8, and C9,
L1 1.6uH
C1
2.2uF
10
POWER OK
U1
SP6121
1
2
7
6
5
4
3
8
Vcc
GND
NDRV
ISET
ISENSE
COMP
VFB
PDRV
GND
C7
0.1uF
U2
SP708
4
6
7
5
1
2
8
3
VCC
PFI
PFO
GND
RESET
Reset
NC
MR
Vin
9
C11
56pF
1,2,3,4
C9
470uF
C8
470uF
R4
10K
TRIM
C6
47uF
D1
R2
2.3K
R3
10K
V out
CERAMIC
7,8
R1
10K
C2
56pF
Q2
FDS6690A
R5
10K
C3
3.9nF
R6
5.0
Q1
FDS6375
C10
0.1uF
5,6
C5
1.0uF
+3.0V-7.0 V
2.5V



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