SC121 [SEMTECH]
Low Voltage Synchronous Boost Regulator; 低电压同步升压稳压器
| 型号： | SC121 |
| 厂家： | 
SEMTECH CORPORATION |
| 描述： | Low Voltage Synchronous Boost Regulator |
| 文件： | 总23页 (文件大小：330K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
SC121
Low Voltage Synchronous
Boost Regulator
POWER MANAGEMENT
Features
Description
Input voltage — 0.7V to 4.5V
The SC121 is a high efficiency, low noise, synchronous
step-up DC-DC converter that provides boosted voltage
levels in low-voltage handheld applications. The wide
input voltage range allows use in systems with single
NiMH or alkaline battery cells as well as in systems with
higher voltage battery supplies. It features an internal
1.2A switch and synchronous rectifier to achieve up to
94% efficiency and to eliminate the need for an external
Schottky diode. The output voltage can be set to 3.3V
with internal feedback, or to any voltage within the speci-
fied range using a standard resistor divider.
Minimum start-up voltage — 0.85V
Output voltage — fixed at 3.3V; adjustable from 1.8V
to 5.0V
Peak input current limit — 1.2A
Output current at 3.3 VOUT — 80mA with VIN = 1.0V,
190mA with VIN = 1.5V
Forced PWM operation at all loads
Efficiency up to 94%
Internal synchronous rectifier
No forward conduction path during shutdown
Switching frequency — 1.2MHz
Soft-start startup current limiting
Shutdown current — 0.1μA (typ)
Ultra-thin 1.5 × 2.0 × 0.6 (mm) MLPD-UT-6 package
Lead-free and halogen-free
The SC121 operates exclusively in Pulse Width Modulation
(PWM) mode for low ripple and fixed-frequency switching.
Output disconnect capability is included to reduce leakage
current, improve efficiency, and eliminate external com-
ponents sometimes needed to disconnect the load from
the supply during shutdown.
WEEE and RoHS compliant
Applications
Low quiescent current is maintained with a high 1.2MHz
operating frequency. Small external components and the
space saving MLPD-UT-6, 1.5×2.0×0.6 (mm) package
make this device an excellent choice for small handheld
applications that require the longest possible battery life.
MP3 players
Smart phones and cellular phones
Palmtop computers and handheld instruments
PCMCIA cards and memory cards
Digital cordless phones
Personal medical products
Wireless VoIP phones
Small motors
Typical Application Circuit
L1
IN
LX
Single
Cell
OUT
3.3V
EN
(1.2V)
FB
GND
CIN
COUT
SC121
April 13, 2010
1
© 2010 Semtech Corporation
SC121
Pin Configuration — MLPD-UT
Ordering Information
Device
Package
SC121ULTRT(1)(2)
MLPD-UT-6 1.5×2
Evaluation Board
SC121EVB
Notes:
1
2
3
LX
GND
IN
6
5
4
OUT
FB
TOP VIEW
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Lead-free packaging, only. Device is WEEE and RoHS compliant,
and halogen-free.
T
EN
MLPD-UT; 1.5×2, 6 LEAD
θJA = 84°C/W
Marking Information — MLPD-UT
121
yw
MLPD-UT; 1.5×2, 6 LEAD
yw = date code
2
SC121
Absolute Maximum Ratings
Recommended Operating Conditions
IN, OUT, LX, FB (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +6.0
EN (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to (VIN + 0.3)
ESD Protection Level(1) (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Ambient Temperature Range (°C). . . . . . . . . . . . -40 to +85
VIN (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.7 to 4.5
VOUT (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 to 5.0
Thermal Information
Thermal Res. MLPD, Junction-Ambient(2) (°C/W) . . . . . . . 84
Maximum Junction Temperature (°C) . . . . . . . . . . . . . . . 150
Storage Temperature Range (°C) . . . . . . . . . . . -65 to +150
Peak IR Reflow Temperature (10s to 30s) (°C) . . . . . . +260
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters
specified in the Electrical Characteristics section is not recommended.
NOTES:
(1) Tested according to JEDEC standard JESD22-A114.
(2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
Electrical Characteristics
Unless otherwise noted VIN = 2.5V, CIN = COUT = 22μF, L1 = 4.7μH, TA = -40 to +85°C. Typical values are at TA = 25°C.
Parameter
Symbol
VIN
Conditions
Min
Typ
Max
Units
V
Input Voltage Range
0.7
4.5
Minimum Startup Voltage
Shutdown Current
VIN-SU
ISHDN
IOUT < 1mA, TA = 0°C to 85°C
TA = 25°C, VEN = 0V
0.85
0.1
3.5
1.2
90
V
1
μA
mA
MHz
%
Operating Supply Current(1)
Internal Oscillator Frequency
Maximum Duty Cycle
Minimum Duty Cycle
Output Voltage
IQ
IOUT = 0, VEN = VIN
fOSC
DMAX
DMIN
20
%
VOUT
VFB = 0V
3.3
V
Adjustable Output Voltage Range
VOUT_RNG
For VIN such that DMIN < D < DMAX
1.8
5.0
1.5
0.1
V
Regulation Feedback Reference Volt-
age Accuracy (Internal or External
Programming)
VReg-Ref
-1.5
%
FB Pin Input Current
Startup Time
IFB
VFB = 1.2V
ꢀA
ms
tSU
1
3
SC121
Electrical Characteristics (continued)
Parameter
Symbol
RDSP
RDSN
ILIM(N)
ILIM(P)-SU
ILXP
Conditions
VOUT = 3.3V
Min
Typ
0.6
Max
Units
Ω
P-Channel ON Resistance
N-Channel ON Resistance
N-Channel Current Limit
P-Channel Startup Current Limit
LX Leakage Current PMOS
LX Leakage Current NMOS
Logic Input High
VOUT = 3.3V
0.5
Ω
VIN = 3.0V
0.9
1.2
A
VIN > VOUT, VEN > VIH
TA = 25°C, VLX = 0V
TA = 25°C, VLX = 3.3V
VIN = 3.0V
150
mA
μA
μA
V
1
1
ILXN
VIH
0.85
-0.2
Logic Input Low
VIL
VIN = 3.0V
0.2
1
V
Logic Input Current High
Logic Input Current Low
IIH
VEN = VIN = 3.0V
VEN = 0V
μA
μA
IIL
NOTES:
(1) Quiescent operating current is drawn from OUT while in regulation. The quiescent operating current projected to IN is approximately
IQ × (VOUT/VIN).
4
SC121
Typical Characteristics — VOUT = 1.8V
Efficiency vs. IOUT (VOUT = 1.8V)
Efficiency vs. IOUT (VOUT = 1.8V)
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, TA = 25οC
Ω
Ω
μ
Ω
Ω
μ
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, VIN = 1.2V
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
VIN = 1.6V
TA = –40°C
TA = 85°C
VIN = 0.8V
TA = 85°C
VIN = 1.2V
TA = –40°C
TA = 25°C
0.1 0.2
0.5
1
2
5
10
20
50 100 200
0.1 0.2
0.5
1
2
5
10
20
50 100 200
IOUT (mA)
IOUT (mA)
Load Regulation (VOUT = 1.8V)
Load Regulation (VOUT = 1.8V)
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, TA = 25οC
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, VIN = 1.2V
Ω
Ω
μ
Ω
Ω
μ
1.82
1.8
1.82
1.8
VIN = 1.6V
TA = –40°C
TA = 25°C
1.78
1.76
1.78
1.76
TA = 85°C
VIN = 1.2V
VIN = 0.8V
0
50
100
150
200
250
0
50
100
150
200
250
IOUT (mA)
IOUT (mA)
Line Regulation — Low Load (VOUT = 1.8V)
Line Regulation — High Load (VOUT = 1.8V)
Ω
Ω
μ
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, IOUT = 1mA
Ω
Ω
μ
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, IOUT = 50mA
1.82
1.8
1.82
1.8
TA = 25°C
TA = 25°C
TA = –40°C
TA = –40°C
TA = 85°C
1.78
1.76
1.78
1.76
TA = 85°C
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
VIN (V)
VIN (V)
5
SC121
Typical Characteristics — VOUT = 1.8V (continued)
Temperature Reg. — Low Load (VOUT = 1.8V)
Temperature Reg. — High Load (VOUT = 1.8V)
Ω
Ω
μ
Ω
Ω
μ
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, IOUT = 50mA
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF, IOUT = 1mA
1.82
1.8
1.82
1.8
VIN = 1.6V
VIN = 1.2V
VIN = 1.2V
VIN = 0.8V
1.78
1.76
1.78
VIN = 0.8V
1.76
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
Junction Temperature (oC)
Junction Temperature (oC)
Max. IOUT vs. VIN (VOUT = 1.8V)
Ω
Ω
μ
R1 = 499k , R2 = 1M , L = 4.7 H, CFB = 22pF
350
300
250
200
150
100
50
TA = –40°C
TA = 25°C
TA = 85°C
0
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
VIN (V)
6
SC121
Typical Characteristics — VOUT = 3.3V
Efficiency vs. IOUT (VOUT = 3.3V)
Efficiency vs. IOUT (VOUT = 3.3V)
FB grounded, L = 4.7 H, TA = 25οC
μ
μ
FB grounded, L = 4.7 H, VIN = 2V
100
90
80
70
60
50
40
30
20
10
0
100
TA = –40°C
90
VIN = 2.95V
80
TA = 85°C
70
VIN = 1.0V
60
VIN = 2.0V
50
40
30
20
10
0
TA = 25°C
0.1 0.2
0.5
1
2
5
10 20
50 100 200
500
0.1 0.2
0.5
1
2
5
10 20
50 100 200
500
IOUT (mA)
IOUT (mA)
Load Regulation (VOUT = 3.3V)
Load Regulation (VOUT = 3.3V)
FB grounded, L = 4.7 H, TA = 25οC
FB grounded, L = 4.7 H, VIN = 2V
μ
μ
3.34
3.32
3.3
3.34
3.32
3.3
TA = 25°C
VIN = 2.95V
3.28
3.26
3.24
3.22
3.2
3.28
3.26
3.24
3.22
3.2
VIN = 2.0V
TA = –40°C
VIN = 1.0V
TA = 85°C
0
50
100 150 200 250 300 350 400 450 500
0
50
100 150 200 250 300 350 400 450 500
IOUT (mA)
IOUT (mA)
Line Regulation — Low Load (VOUT = 3.3V)
Line Regulation — High Load (VOUT = 3.3V)
μ
μ
FB grounded, L = 4.7 H, IOUT = 1mA
FB grounded, L = 4.7 H, IOUT = 90mA
3.34
3.32
3.3
3.34
3.32
3.3
TA = –40°C
TA = –40°C
3.28
3.26
3.24
3.22
3.2
3.28
3.26
3.24
3.22
3.2
TA = 85°C
TA = 85°C
TA = 25°C
TA = 25°C
0.6 0.8
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
3
0.6 0.8
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
3
VIN (V)
VIN (V)
7
SC121
Typical Characteristics — VOUT = 3.3V (continued)
Temperature Reg. — Low Load (VOUT = 3.3V)
Temperature Reg. — High Load (VOUT = 3.3V)
μ
FB grounded, L = 4.7 H, IOUT = 90mA
μ
FB grounded, L = 4.7 H, IOUT = 1mA
3.34
3.32
3.3
3.34
3.32
3.3
VIN = 2.95V
VIN = 2.95V
VIN = 2.0V
VIN = 1.0V
VIN = 2.0V
VIN = 1.0V
3.28
3.26
3.24
3.22
3.2
3.28
3.26
3.24
3.22
3.2
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
Junction Temperature (oC)
Junction Temperature (oC)
Max. IOUT vs. VIN (VOUT = 3.3V)
μ
FB grounded, L = 4.7 H
500
450
400
350
300
250
200
150
100
50
TA = –40°C
TA = 25°C
TA = 85°C
0
0.6 0.8
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
3
VIN (V)
8
SC121
Typical Characteristics — VOUT = 4.0V
Efficiency vs. IOUT (VOUT = 4.0V)
Efficiency vs. IOUT (VOUT = 4.0V)
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, TA = 25οC
Ω
Ω
μ
Ω
Ω
μ
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, VIN = 2.4V
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
TA = –40°C
VIN = 3.6V
TA = 85°C
TA = 25°C
VIN = 1.2V
VIN = 2.4V
0.1 0.2
0.5
1
2
5
10 20
50 100 200
500
0.1 0.2
0.5
1
2
5
10 20
50 100 200
500
IOUT (mA)
IOUT (mA)
Load Regulation (VOUT = 4.0V)
Load Regulation (VOUT = 4.0V)
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, TA = 25οC
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, VIN = 2.4V
Ω
Ω
μ
Ω
Ω
μ
4.1
4.05
4
4.1
4.05
4
VIN = 3.6V
TA = 25°C
3.95
3.9
3.95
3.9
VIN = 1.2V
VIN = 2.4V
TA = –40°C
TA = 85°C
3.85
3.85
0
50
100 150 200 250 300 350 400 450 500 550
0
50
100 150 200 250 300 350 400 450 500 550
IOUT (mA)
IOUT (mA)
Line Regulation — Low Load (VOUT = 4.0V)
Line Regulation — High Load (VOUT = 4.0V)
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, IOUT = 110mA
Ω
Ω
μ
Ω
Ω
μ
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, IOUT = 1mA
4.1
4.05
4
4.1
4.05
4
TA = 85°C
TA = –40°C
TA = –40°C
TA = 85°C
3.95
3.9
3.95
3.9
TA = 25°C
TA = 25°C
3.85
3.85
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
VIN (V)
VIN (V)
9
SC121
Typical Characteristics — VOUT = 4.0V (continued)
Temperature Reg. — High Load (VOUT = 4.0V)
Temperature Reg. — Low Load (VOUT = 4.0V)
Ω
Ω
μ
Ω
Ω
μ
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, IOUT = 110mA
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF, IOUT = 1mA
4.1
4.05
4
4.1
4.05
4
VIN = 3.6V
VIN = 3.6V
VIN = 2.4V
VIN = 1.2V
VIN = 1.2V
VIN = 2.4V
3.95
3.9
3.95
3.9
3.85
3.85
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
Junction Temperature (oC)
Junction Temperature (oC)
Max. IOUT vs. VIN (VOUT = 4.0V)
Ω
Ω
μ
R1 = 976k , R2 = 412k , L = 4.7 H, CFB = 22pF
500
450
400
350
300
250
200
150
100
50
TA = –40°C
TA = 85°C
TA = 25°C
0
0.8
1.2
1.6
2
2.4
2.8
3.2
3.6
VIN (V)
10
SC121
Typical Characteristics — VOUT = 5.0V
Efficiency vs. IOUT (VOUT = 5.0V)
Efficiency vs. IOUT (VOUT = 5.0V)
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, TA = 25οC
Ω
Ω
μ
Ω
Ω
μ
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, VIN = 3.6V
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
VIN = 4.2V
TA = –40°C
TA = 85°C
VIN = 1.2V
VIN = 2.2V
TA = 25°C
VIN = 3.2V
0.1 0.2
0.5
1
2
5
10 20
50 100 200
500
0.1 0.2
0.5
1
2
5
10 20
50 100 200
500
IOUT (mA)
IOUT (mA)
Load Regulation (VOUT = 5.0V)
Load Regulation (VOUT = 5.0V)
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, TA = 25οC
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, VIN = 3.6V
Ω
Ω
μ
Ω
Ω
μ
5.05
5
5.05
5
VIN = 4.2V
TA = –40°C
4.95
4.9
4.85
4.8
4.95
4.9
4.85
4.8
TA = 85°C
TA = 25°C
VIN = 3.2V
VIN = 1.2V
VIN = 2.2V
0
50
100 150 200 250 300 350 400 450 500 550
0
50
100 150 200 250 300 350 400 450 500 550
IOUT (mA)
IOUT (mA)
Line Regulation — Low Load (VOUT = 5.0V)
Line Regulation — High Load (VOUT = 5.0V)
Ω
Ω
μ
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, IOUT = 1mA
Ω
Ω
μ
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, IOUT = 85mA
5.05
5
5.05
5
TA = –40°C
TA = –40°C
4.95
4.9
4.85
4.8
4.95
4.9
4.85
4.8
TA = 85°C
TA = 25°C
TA = 85°C
TA = 25°C
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0.5
1
1.5
2
2.5
3
3.5
4
4.5
VIN (V)
VIN (V)
11
SC121
Typical Characteristics — VOUT = 5.0V (continued)
Temperature Reg. — Low Load (VOUT = 5.0V)
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, IOUT = 1mA
Temperature Reg. — High Load (VOUT = 5.0V)
Ω
Ω
μ
Ω
Ω
μ
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF, IOUT = 85mA
5.05
5
5.05
5
VIN = 4.2V
VIN = 4.2V
VIN = 3.2V
4.95
4.9
4.85
4.8
4.95
4.9
4.85
4.8
VIN = 2.2V
VIN = 1.2V
VIN = 3.2V
VIN = 2.2V
VIN = 1.2V
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
Junction Temperature (oC)
Junction Temperature (oC)
Max. IOUT vs. VIN (VOUT = 5.0V)
Ω
Ω
μ
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF
500
450
400
350
300
250
200
150
100
50
TA = –40°C
TA = 25°C
TA = 85°C
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
VIN (V)
12
SC121
Typical Characteristics (continued)
PWM Operation
Load Transient
VOUT = 3.3V, VIN = 1.5V, TA =25°C
VOUT = 3.3V, VIN = 1.5V, IOUT = 50mA
VOUT ripple
(10mV/div)
IOUT = 40mA to
140mA
(50mA/div)
IL
(100mA/div)
VOUT
(100mV/div)
AC Coupled
VLX
(5V/div)
Time = (100μs/div)
Time = (400ns/div)
Startup Min Load Res. vs. VIN (Any VOUT
)
Startup Max Load Current vs. VIN (Any VOUT
)
Ω
Ω
μ
Ω
Ω
μ
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF
R1 = 931k , R2 = 294k , L = 4.7 H, CFB = 22pF
100
80
60
40
20
0
160
140
120
100
80
TA = –40°C
TA = 25°C
TA = –40°C
TA = 85°C
60
40
TA = 25°C
TA = 85°C
20
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0.5
1
1.5
2
2.5
3
3.5
4
4.5
VIN (V)
VIN (V)
Min. Start-up Voltage vs. Temperature (Any VOUT
)
VOUT = 3.3V, IOUT = 1mA
0.9
0.85
0.8
0.75
0.7
0.65
0.6
-40
0
20
40
60
-20
80
100
Temperature (°C)
13
SC121
Pin Descriptions
MLPD Pin #
Pin Name
Pin Function
1
2
LX
Switching node — connect an inductor from the input supply to this pin.
Signal and power ground.
GND
Battery or supply input — requires an external 10μF bypass capacitor (capacitance evaluated while under
IN bias) for normal operation.
3
IN
V
4
5
EN
FB
Enable digital control input — active high.
Feedback input — connect to GND for preset 3.3V output. A voltage divider is connected from OUT to GND
to adjust output from 1.8V to 5.0V.
Output voltage pin — requires an external 10μF bypass capacitor (capacitance evaluated while under VOUT
bias) for normal operation.
6
T
OUT
Thermal
Pad
Thermal Pad is for heat sinking purposes — connect to ground plane using multiple vias — not connected
internally.
14
SC121
Block Diagram
VOUT
Comp.
IN
+
-
OUT
+
1.7 V
-
+
-
Start-up
Oscillator
EN
P
LIM
Amp.
Gate Drive
and
Logic
Bulk
Bias
Oscillator and
Slope
Generator
Slope
Comp.
Control
LX
PWM
Comp.
+
PWM
Control
-
+
-
NLIM
Amplifier
Error
+
-
Amp.
Output Voltage
Selection Logic
-
FB
Current
Amplifier
+
GND
VREF
+
-
1.2 V
15
SC121
Applications Information
The values of the resistors in the voltage divider network
are chosen to satisfy the equation
Detailed Description
The SC121 is a synchronous step-up fixed frequency Pulse
Width Modulated (PWM) DC-DC converter utilizing a
1.2MHz fixed frequency current mode architecture. It is
designed to provide output voltages in the range 1.8V to
5.0V from an input voltage as low as 0.7V, with a (output
unloaded) start up input voltage of 0.85V. Quiescent
current consumption is typically 3.5mA, entirely into the
OUT pin during boost regulation. (See footnote 1 of the
Electrical Characteristics table.)
§
·
R1
R2
¨
¸
¸
VOUT 1.191u 1ꢂ
V
ꢀ ꢁ
¨
©
¹
A large value of R2, ideally 590kΩ or larger, is preferred for
stability for VIN within approximately 400mV of VOUT. For
lower VIN, lower resistor values can be used. The values of
R1 and R2 can be as large as desired to achieve low quies-
cent current. CFB = 22pF is recommended to improve
transient response.
The regulator control circuitry is shown in the Block
Diagram. It is comprised of a programmable feedback
controller, an internal 1.2MHz oscillator, an n-
channel Field Effect Transistor (FET) between the LX and
GND pins, and a p-channel FET between the LX and OUT
pins. The current flowing through both FETs is monitored
and limited as required for startup and PWM operation.
An external inductor must be connected between the IN
pin and the LX pin.
The Enable Pin
The EN pin is a high impedance logical input that can be
used to enable or disable the SC121 under processor
control. VEN < 0.2V will disable regulation, set the LX pin
in a high-impedance state (turn off both FET switches),
and turn on an active discharge device to discharge the
output capacitor via the OUT pin. Synchronous rectifier
(p-channel FET) bulk switching prevents pass-through
conduction from LX to OUT while disabled. VEN > 0.85V
will enable the output. The startup sequence from the
EN pin is identical to the startup sequence from the appli-
cation of input power.
Output Voltage Selection
The SC121 output voltage can be programmed to an
internally preset value or it can be programmed with
external resistors. The output is internally programmed to
3.3V when the FB pin is connected to GND. Any output
voltage in the range 1.8V to 5.0V can be programmed with
a resistor voltage divider between OUT and the FB pin as
shown in Figure 1.
L1
IN
LX
OUT
EN
VOUT
CFB
R1
FB
GND
CIN
COUT
SC121
R2
Figure 1 — Output Voltage Feedback Circuit
16
SC121
Applications Information (continued)
The minimum on-time limitation imposes a minimum
boost ratio, so if VIN is too close to VOUT (VIN > VOUT – 400mV,
approximately), VOUT will rise above the programmed
value for a sufficiently small output load. A higher output
load requires a higher duty cycle to overcome dissipative
losses, such that regulation at programmed VOUT will
eventually be restored. But this regulation-restoration
load rises rapidly with VIN, so this phenomenon can be
beneficially exploited in only rare circumstances. If opera-
tion with high VIN and low load is required, please consider
using the SC120, a pin compatible dual mode (PWM/
PSAVE) boost converter. The SC120 will support zero load
in PSAVE mode for VIN up to VOUT + 150mV.
PWM Operation
The PWM cycle runs at a fixed frequency (fosc = 1.2MHz),
with a variable duty cycle (D). PWM operation continually
draws current from the input supply, except for low
output loads in which current flows periodically from, and
back into, the input. During the on-state of the PWM
cycle, the n-channel FET is turned on, grounding the
inductor at the LX pin. This causes the current flowing
from the input supply through the inductor to ground to
ramp up. During the off-state, the n-channel FET is turned
off and the p-channel FET (synchronous rectifier) is turned
on. This causes the inductor current to flow from the
input supply through the inductor into the output capaci-
tor and load, boosting the output voltage above the input
voltage. The cycle then repeats to re-energize the
inductor.
Regulator Startup, Short Circuit Protection,
and Current Limits
The SC121 permits power up at input voltages from 0.85V
to 4.5V. Soft-start startup current limiting of the internal
switching n-channel and p-channel FET power devices
protects them from damage in the event of a short
between OUT and GND. As the output voltage rises, pro-
gressively less-restrictive current limits are applied. This
protection unavoidably prevents startup into an exces-
sive load.
Ideally, the steady state (constant load) duty cycle is
determined by D = 1 – (VIN/VOUT), but must be greater in
practice to overcome dissipative losses. The SC121 PWM
controller constrains the value of D such that 0.20 < D < 0.90
(approximately).
The average inductor current during the off-state multi-
plied by (1-D) is equal to the average load current. The
inductor current is alternately ramping up (on-state) and
down (off-state) at a rate and amplitude determined by
the inductance value, the input voltage, and the on-time
(TON = D×T, T = 1/fOSC). Therefore, the instantaneous induc-
tor current will be alternately larger and smaller than the
average.
Upon enable, the p-channel FET between the LX and OUT
pins turns on with its current limited to approximately
150mA, the short-circuit output current. When VOUT
approaches VIN (but is still below 1.7V), the n-channel
current limit is set to 350mA (the p-channel limit is dis-
abled), the internal oscillator turns on (approximately
200kHz), and a fixed 75% duty cycle PWM operation
begins. (See the section PWM Operation.) When the
output voltage exceeds 1.7V, fixed frequency PWM opera-
tion begins, with the duty cycle determined by an n-
channel FET peak current limit of 350mA. When this
n-channel FET startup current limit is exceeded, the on-
state ends immediately and the off-state begins. This
determines the duty cycle on a cycle-by-cycle basis.
When VOUT is within 2% of the programmed regulation
voltage, the n-channel FET current limit is raised to 1.2A,
and normal voltage regulation PWM control begins.
If the average output current is sufficiently small, the
minimum inductor current can ramp down to zero during
the off-state. Discontinuous mode operation (where both
FETs turn off as the inductor current reaches zero) is not
supported in the SC121, since this would result in a finite
positive minimum current from input to output, which
would cause an uncontrolled rise in output voltage in this
case. Instead, the inductor current will reverse for the
remainder of the off-state, flowing from the output
capacitor into the OUT pin, through the p-channel FET to
the LX pin, and through the inductor to the input capaci-
tor. Negative inductor current ripple allows regulation
even with zero output load. The energy returned to the
input capacitor is not wasted, but dissipative conduction
losses will inevitably occur.
Once normal voltage regulation PWM control is initiated,
the output becomes independent of VIN and output regu-
lation can be maintained for VIN as low as 0.7V, subject to
the maximum duty cycle and peak current limits. The
17
SC121
Applications Information (continued)
Once an overload has occurred, the load must be
decreased to permit recovery. The conditions required for
overload recovery are identical to those required for suc-
cessful initial startup.
duty cycle must remain between 20% and 90% for the
device to operate within specification.
Note that startup with a regulated active load is not the
same as startup with a resistive load. The resistive load
output current increases proportionately as the output
voltage rises until it reaches programmed VOUT/RLOAD, while
a regulated active load presents a constant load as the
output voltage rises from 0V to programmed VOUT. Note
also that if the load applied to the output exceeds an
applicable VOUT–dependent startup current limit or duty
cycle limit, the criterion to advance to the next startup
stage may not be achieved. In this situation startup may
pause at a reduced output voltage until the load is reduced
further.
Component Selection
The SC121 provides optimum performance when a 4.7μH
inductor is used with a 10μF output capacitor. Different
component values can be used to modify input current or
output voltage ripple, improve transient response, or to
reduce component size or cost.
Inductor Selection
The inductance value primarily affects the amplitude of
inductor peak-to-peak current ripple (ΔIL). Reducing
inductance increases ΔIL and raises the inductor peak
current, IL-max = IL-avg + ΔIL/2, where IL-avg is the inductor
current averaged over a full on/off cycle. IL-max is subject to
the n-channel FET current limit ILIM(N), therefore reducing
the inductance may lower the output overload current
threshold. Increasing ΔIL also lowers the inductor
minimum current, IL-min = IL-avg – ΔIL/2, thus raising the load
current threshold below which inductor negative–peak
current becomes zero.
Output Overload and Recovery
The PWM steady state duty cycle is determined by
D = 1 – (VIN/VOUT), but must be somewhat greater in prac-
tice to overcome dissipative losses. As the output load
increases, the dissipative losses also increase. The PWM
controller must increase the duty cycle to compensate.
Eventually, one of two overload conditions will occur,
determined by VIN, VOUT, and the overall dissipative losses
due to the output load current. Either the maximum duty
cycle of 90% will be reached or the n-channel FET 1.2A
(nominal) peak current limit will be reached, which effec-
tively limits the duty cycle to a lower value. Above that
load, the output voltage will decrease rapidly and in
reverse order the startup current limits will be invoked as
the output voltage falls through its various voltage thresh-
olds. How far the output voltage drops depends on the
load voltage vs. current characteristic.
Equating input power to output power and noting that
input current is equal to inductor current, average the
inductor current over a full PWM switching cycle to
obtain
1 VOUT uIOUT
ILꢃavg
u
K
V
IN
where η is efficiency.
A reduction in input voltage, such as a discharging battery,
will lower the load current at which overload occurs.
Lower input voltage increases the duty cycle required to
produce a given output voltage. And lower input voltage
also increases the input current to maintain the input
power, which increases dissipative losses and further
increases the required duty cycle. Therefore an increase in
load current or a decrease in input voltage can result in
output overload. Please refer to the Max. IOUT vs. VIN Typical
Characteristics plots for the condition that best matches
the application.
Neglecting the n-channel FET RDS-ON and the inductor DCR,
for duty cycle D, and with T = 1/fosc,
DT
1
L
V uDuT
IN
'ILꢃon
V dt
IN
³
0
L
This is the change in IL during the on-state. During the
off-state, again neglecting the p-channel FET RDS-ON and
the inductor DCR,
T ꢀV ꢃ VOUT
ꢁ
dt
ꢀ
1ꢃD
ꢁ
1
L
ꢀ
V ꢃ VOUT
ꢁ
uT
IN
'ILꢃoff
IN
³
DT
L
18
SC121
Applications Information (continued)
Note that this is a negative quantity, since VOUT > VIN and
0 < D < 1. For a constant load in steady-state, the inductor
current must satisfy ΔIL-on + ΔIL-off = 0. Substituting the two
Rated
Current
(mA)
Dimensions
LxWxH
Manufacturer/
Part #
Value DCR
Tolerance
(%)
(ꢀH)
(Ω)
(mm)
Murata
LQM31PN4R7M00
expressions and solving for D, obtain D = 1 – VIN/VOUT
.
4.7
4.7
0.3
0.7
700
500
20
20
3.2 x 1.6 x 0.95
2 x 2 x 0.6
Using this expression, and the positive valued expression
ΔIL = ΔIL-on for current ripple amplitude, obtain expanded
Coilcraft
XFL2006-472
expression for IL-max and IL-min
.
Capacitor Selection
VOUT uIOUT
T
V
Input and output capacitors must be chosen carefully to
ensure that they are of the correct value and rating. The
output capacitor requires a minimum capacitance value
of 10μF at the programmed output voltage to ensure sta-
bility over the full operating range. This must be consid-
ered when choosing small package size capacitors as the
DC bias must be included in their derating to ensure this
required value. For example, a 10μF 0805 capacitor may
provide sufficient capacitance at low output voltages but
may be too low at higher output voltages. Therefore, a
higher capacitance value may be required to provide the
minimum of 10μF at these higher output voltages.
IN
ILꢃmax,min
r
u
u
ꢀ
VOUT ꢃ VIN ꢁ
V uK
2uL VOUT
IN
From this result, obtain an alternative expression for ΔIL.
T
L
V
VOUT
IN
'IL ILꢃmax ꢃILꢃmin
u
u
ꢀ
VOUT ꢃ VIN ꢁ
The inductor selection should consider the n-channel FET
current limit for the expected range of input voltage and
output load current. The largest IL-avg will occur at the
expected smallest VIN and largest IOUT. Determine the
largest expected ΔIL. Then for the largest expected IL-avg
,
ensure that the n-channel FET current limit is not exceed.
That is, for the minimum n-channel FET current limit,
worst case inductor tolerance, highest expected output
current, and lowest expected VIN, ensure that
Low ESR capacitors such as X5R or X7R type ceramic
capacitors are recommended for input bypassing and
output filtering. Low-ESR tantalum capacitors are not
recommended due to possible reduction in capacitance
seen at the switching frequency of the SC121. Ceramic
capacitors of type Y5V are not recommended as their tem-
perature coefficients make them unsuitable for this appli-
cation. The following table lists recommended capacitors.
For smaller values and smaller packages, it may be neces-
sary to use multiples devices in parallel.
IL-max = IL-avg + ΔIL/2 < ILIM(N)
.
Many of these equations include the parameter η, effi-
ciency. Efficiency varies with VIN, IOUT, and temperature.
Estimate η using the plots provided in this datasheet, or
from experimental data, at the operating condition of
interest.
Case
Height
(mm)
Manufacturer/
Part Number
Value
(ꢀF)
Rated Volt-
age (VDC)
Case
Size
Type
Any chosen inductor should have low DCR compared to
the RDS-ON of the FET switches to maintain efficiency,
though for DCR << RDS-ON, further reduction in DCR will
provide diminishing benefit. The inductor ISAT value should
exceed the expected IL-max. The inductor self-resonant fre-
quency should exceed 5×fosc. Any inductor with these
properties should provide satisfactory performance.
L = 4.7μH should perform well for most applications.
Murata
GRM21BR60J226ME39B
22
22
4.7
22
22
6.3
10
4
X5R
X7R
X5R
X5R
X5R
0805
1206
0603
0805
0805
1.25
1.6
Murata
GRM31CR71A226KE15L
Murata
GRM185R60G475ME15
0.5
TDK
10
20
0.85
1.25
C2012X5R1A226M
Taiyo Yuden
JMK212BJ226MG-T
The following table lists the manufacturers of recom-
mended inductor options. The specification values shown
are simplified approximations or averages of many device
parameters under various test conditions. See manufac-
turers’documentation for full performance data.
19
SC121
Applications Information (continued)
• Maximize ground metal on the component side
PCB Layout Considerations
Poor layout can degrade the performance of the DC-DC
converter and can contribute to EMI problems, ground
bounce, and resistive voltage losses. Poor regulation and
instability can result.
to improve the return connection and thermal
dissipation. Separation between the LX node
and GND should be maintained to avoid cou-
pling capacitance between the LX node and the
ground plane.
The following simple design rules can be implemented to
ensure good layout:
• Use a ground plane with several vias connecting
to the component side ground to further reduce
noise interference on sensitive circuit nodes.
• Place the inductor and filter capacitors as close
to the device as possible and use short wide
traces between the power components.
• Route the output voltage feedback path away
from the inductor and LX node to minimize
noise and magnetic interference.
A suggested layout is shown in Figure 4.
7.0mm
VOUT
COUT
LX
OUT
CFB
R1
FB
EN
GND
SC121
5.2mm
LX
(2nd layer)
IN
R2
CIN
GND
VIN
Figure 4 — Layout Drawing
20
SC121
Outline Drawing — MLPD-UT-6 1.5x2
DIMENSIONS
INCHES
B
E
A
D
MILLIMETERS
DIM
MIN
.020
.000
NOM
-
MAX
.024
.002
MIN
0.50
0.00
NOM MAX
-
A
A1
A2
b
0.60
-
-
0.05
(.006)
.010
(.152)
0.25
PIN 1
INDICATOR
.007
.055
.035
.075
.026
.012
.063
.055
.083
.035
0.18
1.40
0.90
1.90
0.65
0.30
1.60
1.40
2.10
0.90
D
.059
-
1.50
-
(LASER MARK)
D1
E
.079
.031
2.00
0.80
E1
e
.020 BSC
.014
0.50 BSC
0.35
A2
C
L
N
aaa
.012
.016
0.30
0.40
A
6
6
SEATING
PLANE
.003
0.08
aaa
C
bbb
.004
0.10
A1
D1
2
1
LxN
E1
N
bxN
bbb
C A B
e
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS TERMINALS.
21
SC121
Land Pattern — MLPD-UT-6 1.5x2
H
R
DIMENSIONS
INCHES
DIM
MILLIMETERS
(.077)
.047
.051
.031
.020
.006
.012
.030
.106
(1.95)
1.20
1.30
0.80
0.50
0.15
0.30
0.75
2.70
C
G
H
K
P
R
X
Y
Z
Z
G
Y
(C)
K
P
X
NOTES:
1.
2.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
3.
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
22
SC121
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