SSC SS6638

SS6638G
Simple 3-Pin Step-Up DC/DC Converter
FEATURES
DESCRIPTION
Guaranteed start-up from less than 0.9 V.
High efficiency.
Low quiescent current.
Fewer external components needed.
Low ripple and low noise.
Fixed output voltage: 2.7V, 3.0V, 3.3V, 4.5V
and 5V.
Space saving packages: SOT-23, SOT-89
and TO-92.
The SS6638G is a high-efficiency step-up DC/DC
converter for applications using 1 to 4 NiMH
battery cells. Only three external components are
required to deliver a fixed output voltage of 2.7V,
3.0V, 3.3V, 4.5V or 5V. The SS6638G starts up
from less than 0.9V input with 1mA load. A Pulse
Frequency Modulation scheme brings optimized
performance for applications with light output
loading and low input voltages. The output ripple
and noise are lower compared with circuits
operating in PSM mode.
Pb-free, RoHS compliant.
APPLICATIONS
The PFM control circuit operating at a maximum
100kHz switching rate results in smaller passive
components. The space saving SOT-23, SOT-89
and TO-92 packages make the SS6638G an ideal
choice of DC/DC converter for space-conscious
applications, like pagers, electronic cameras, and
wireless microphones.
Pagers.
Cameras.
Wireless Microphones.
Pocket Organizers.
Battery Backup Supplies.
Portable Instruments.
TYPICAL APPLICATION CIRCUIT
VIN
VOUT
L1
100µH
+
C1
SW
22µF
D1
SS12
SS6638-27G
SS6638-30G
SS6638-33G
SS6638-45G
SS6638-50G
VOUT
+ C2
47µF
GND
Simple Step-Up DC/DC Converter
8/21/2005 Rev.2.3
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SS6638G
ORDERING INFORMATION
PIN CONFIGURATION
SS6638-XX X X XX
Packing
TR: Tape and reel
Package type
X: SOT-89
Z: TO-92
U: SOT-23
G: Pb-free, RoHS-compliant
Output voltage
27: 2.7V
30: 3.0V
33: 3.3V
45: 4.5V
50: 5.0V
Example: SS6638-27GXTR
2.7V output in RoHS-compliant SOT-89,
shipped on tape and reel
SOT-89
TOP VIEW
1: GND
2: VOUT
3: SW
1
TO-92
TOP VIEW
1: GND
2: VOUT
3: SW
2
3
1
2
3
SOT-23
TOP VIEW
1: GND
2: VOUT
3: SW
2
1
3
SOT-23 MARKING
Part No.
SS6638-27GU
DA27P
SS6638-30GU
DA30P
SS6638-33GU
DA33P
SS6638-45GU
DA45P
SS6638-50GU
DA50P
SOT-89 MARKING
Part No.
SS6638-27GX
AN27P
SS6638-30GX
AN30P
SS6638-33GX
AN33P
SS6638-45GX
AN45P
SS6638-50GX
AN50P
8/21/2005 Rev.2.3
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SS6638G
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VOUT pin)
.6V
SW pin Voltage
6V
SW pin Switch Current
0.6A
-40°C to 85°C
Operating Temperature Range
Maximum Junction Temperature
125°C
-65°C to 150 °C
Storage Temperature Range
260°C
Lead Temperature (Soldering 10 Sec.)
Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
TEST CIRCUIT
VIN
IIN
VOUT
L1
100µH
+
C1
SW
22µF
D1
SS12
SS6638-27G
SS6638-30G
SS6638-33G
SS6638-45G
VOUT
SS6638-50G
+ C2
47µF
GND
Fig. 1 Test Circuit 1
IS
SS6638G
SS6638G
100
VS
VOUT
SW
VSW
VS
VOUT
FOSC
GND
GND
Fig. 2 Test Circuit 2
8/21/2005 Rev.2.3
SW
Fig. 3 Test Circuit 3
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SS6638G
ELECTRICAL CHARACTERISTICS
PARAMETER
Output Voltage
TEST CONDITIONS
(TA=25°C, IOUT=10mA, unless otherwise
specified) (Note1)
TEST
CKT
SYMBOL
TYP.
MAX.
SS6638-27G
VIN=1.8V
2.633
2.700
2.767
SS6638-30G
VIN=1.8V
2.925
3.000
3.075
SS6638-33G
VIN=2.0V
3.218
3.300
3.382
SS6638-45G
VIN=3.0V
4.387
4.500
4.613
SS6638-50G
VIN=3.0V
4.875
5.000
5.125
1
VOUT
Input Voltage
Normal Operation
1
VIN
Start-Up Voltage
IOUT=1mA, VIN:0→2V
1
VSTART
Min. Hold-on Voltage
IOUT=1mA, VIN:2→0V
1
VHOLD
No-Load Input Current
IOUT=0mA
1
IIN
Supply Current
MIN.
0.8
42
SS6638-30G
50
SS6638-33G
60
SS6638-45G
2
SS6638-50G
IS1
V
6
V
0.9
V
0.7
V
µA
15
SS6638-27G
UNIT
70
µA
90
VS=VOUT x 0.95
Measurement of the IC
input current (VOUT pin)
Supply Current
SS6638-27G
7
SS6638-30G
7
SS6638-33G
7
SS6638-45G
2
IS2
SS6638-50G
7
µA
7
VS=VOUT + 0.5V
Measurement of the IC
input current (VOUT pin)
SW Leakage Current
SW Switch-On
Resistance
VSW=6V, VS=VOUT + 0.5V
0.5
2
SS6638-27G
1.3
SS6638-30G
1.2
SS6638-33G
1.1
SS6638-45G
2
RON
SS6638-50G
1
µA
Ω
1
VS=VOUT x 0.95,
VSW=0.4V
8/21/2005 Rev.2.3
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SS6638G
ELECTRICAL CHARACTERISTICS
PARAMETER
(Continued)
TEST CONDITIONS
TEST
CKT
SYMBOL
MIN.
TYP.
MAX.
UNIT
3
DUTY
65
75
85
%
3
FOSC
80
105
130
kHz
1
η
VS=VOUT x 0.95
Oscillator Duty Cycle
Measurement of the SW
pin waveform
VS=VOUT x 0.95
Max. Oscillator Freq.
Measurement of the SW
pin waveform
Efficiency
85
%
Note 1: Specifications are production tested at TA=25°C. Specifications over the -40°C to 85°C operating
temperature range are assured by design, characterization and correlation with Statistical Quality
Controls (SQC).
TYPICAL PERFORMANCE CHARACTERISTICS (Refer to Typical Application)
2.8
85
2.7
80
2.6
VIN =1.8V
VIN =1.5V
VIN =2.0V
Efficiency (%)
Output Voltage (V)
Capacitor (C2) : 47µF (Tantalum Type)
Diode
(D1) : 1N5819 Schottky Type
V IN =1.2V
2.5
2.4
75
VIN=1.8V
70
VIN=2.0V
65
VIN=1.5V
V IN =0.9V
60
2.3
VIN=1.2V
VIN=0.9V
2.2 0
55
20
40
60
80
100
120
140
160
180
0
20
40
60
80
100
120
140
160
180
Output current (mA)
Output Current (mA)
Fig. 5 SS6638-27G Efficiency (L=100µH CD54)
Fig. 4 SS6638-27G Load Regulation (L=100µH CD54)
2.8
85
80
2.7
Efficiency (%)
Output Voltage (V)
75
2.6
VIN=1.5V
VIN=1.2V
VIN=1.8V
VIN=2.0V
2.5
70
VIN=2.0V
VIN=1.8V
65
60
VIN=1.2V
2.4
VIN=1.5V
VIN=0.9V
55
VIN=0.9V
2.3
0
20
40
60
80
100
120
140
160
180
200
220
Output Current (mA)
Fig. 6
8/21/2005 Rev.2.3
SS6638-27G Load Regulation (L=47µH CD54)
240
50
0
20
40
60
80
100
120
140
160
180
200
220 240
Output current (mA)
Fig. 7 SS6638-27G Efficiency (L=47µH CD54)
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SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
1.0
1.0
0.9
0.9
0.8
Start up
Input Voltage (V)
Input Voltage (V)
0.6
0.5
Hold on
0.4
Start up
0.8
0.7
0.3
0.2
0.7
0.6
0.5
Hold on
0.4
0.3
0.2
0.1
0.1
0.0
0
2
4
6
8
10
12
14
16
0.0
18
0
2
4
6
8
10
12
14
16
18
Output Current (mA)
Output Current (mA)
Fig. 8 SS6638-27G Start-Up & Hold-ON Voltage (L=47µH CD54)
Fig. 9 SS6638-27G Start-Up & Hold-ON Voltage (L=100µH CD54)
2.80
160
Switching Frequency (kHz)
2.78
2.76
Output Voltage (V)
2.74
2.72
2.70
2.68
2.66
2.64
140
120
100
80
60
2.62
2.60
-40
-20
0
20
40
60
80
40
-40
100
Temperature (°C)
Fig. 10 SS6638-27G Output Voltage vs. Temperature
0
20
40
60
80
100
Temperature (°C)
Fig. 11 SS6638-27G Switching Frequency vs. Temperature
80
1.8
SW Turn ON Resistance (Ω)
Maximum Duty Cycle (%)
-20
78
76
74
72
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
70
-40
0.0
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 12 SS6638-27G Maximum Duty Cycle vs. Temperature
8/21/2005 Rev.2.3
-40
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 13 SS6638-27G SW Turn ON Resistance vs. Temperature
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SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
45
3.1
VIN=2.0V
3.0
40
Output voltage VOUT(V)
Supply Current (µA)
2.9
35
30
25
20
15
VIN=1.5V
2.8
VIN=1.8V
2.7
2.6
2.5
2.4
2.3
VIN=1.2V
2.2
10
VIN=0.9V
2.1
5
-40
-20
0
20
40
60
80
2.0
100
0
Temperature (°C)
Fig. 14 SS6638-27G Supply Current vs. Temperature
10
20
30
40
50
60
70
80
90
100 110 120 130 140
Output Current (mA)
Fig. 15 SS6638-30G Load Regulation (L=100µH, CD54)
85
3.1
3.0
80
2.9
Output Voltage (V)
Efficiency (%)
75
70
VIN=2.0
VIN=1.8V
65
60
VIN=1.2V
2.8
VIN=2.0V
VIN=1.8V
VIN=1.5V
2.7
2.6
2.5
VIN=1.5V
2.4
55
2.3
VIN=1.2V
VIN=0.9V
2.2
50
0
20
40
60
80
100
120
140
160
180
VIN=0.9V
0
20
40
60
80
100
120
140
160
180
200
220
Output Current (mA)
Output Current (mA)
Fig. 17 SS6638-30G Load Regulation (L=47µH CD54)
Fig. 16 SS6638-30G Efficiency (L=100µH, CD54)
85
1.0
Start up
0.9
80
0.8
Input Voltage (V)
Efficiency (%)
75
70
65
VIN=2.0V
VIN=1.8V
60
0.7
0.6
Hold on
0.5
0.4
0.3
0.2
55
VIN=1.5V
VIN=0.9V
50
0
25
0.1
VIN=1.2V
50
75
100
125
150
175
200
225
0.0
0
Output Current (mA)
Fig. 18
8/21/2005 Rev.2.3
SS6638-30G Efficiency (L=47µH CD54)
Fig. 19
2
4
6
8
10
12
14
16
18
20
Output Current (mA)
SS6638-30G Start-up & Hold-on Voltage (L=100µH CD54)
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7 of 19
SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
3.10
1.0
3.08
Start up
0.9
3.06
Output Voltage (V)
Input Voltage (V)
0.8
0.7
0.6
0.5
Hold on
0.4
0.3
3.02
3.00
2.98
2.96
0.2
2.94
0.1
2.92
0.0
0
2
4
6
8
10
12
14
16
18
No Load
3.04
2.90
-40
20
-20
Output Current (mA)
Fig. 20
SS6638-30G Start-up & Hold-on Voltage (L=47µH CD54)
Fig. 21
0
20
40
60
80
100
Temperature (°C)
SS6638-30G Output Voltage vs. Temperature
80
Maximum Duty Cycle (%)
Switching Frequency (kHz)
160
140
120
100
80
60
40
-40
Fig. 22
-20
0
20
40
60
80
76
74
72
70
-40
100
Temperature (°C)
SS6638-30G Switching Frequency vs. Temperature
Fig. 23
-20
0
20
40
60
80
100
Temperature (°C)
SS6638-30G Maximum Duty Cycle vs. Temperature
45
1.8
1.6
40
1.4
35
Supply Current (µA)
SW Turn ON Resistance (Ω)
78
1.2
1.0
0.8
0.6
30
25
20
15
0.4
10
0.2
0.0
-40
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 24 SS6638-30G SW Turn ON Resistance vs. Temperature
8/21/2005 Rev.2.3
5
-40
Fig. 25
-20
0
20
40
60
80
100
Temperature (°C)
SS6638-30G Supply Current vs. Temperature
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8 of 19
SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
90
3.4
VIN=2.0V
3.3
85
3.1
80
VIN=1.8V
VIN=1.5V
Efficiency (%)
Output Voltage (V)
3.2
3.0
2.9
VIN=1.2V
2.8
2.7
2.6
VIN=2.0V
75
70
VIN=1.8V
65
60
VIN=1.2V
2.5
2.4
2.3
0
25
50
VIN=0.9V
75
100
125
150
175
50
200
0
25
50
75
Output Current (mA)
SS6638-33G Load Regulation (L=100µH, CD54)
Fig. 27
3.4
90
3.3
85
3.2
80
3.1
VIN=1.5V
VIN=2.0V
VIN=1.8V
3.0
2.9
2.8
2.7
VIN=0.9V
0
25
Fig. 28
50
75
100
125
150
175
200
200
VIN=2.0V
70
65
60
40
225
VIN=1.8V
VIN=1.5V
VIN=0.9V
0
VIN=1.2V
25
50
75
100
125
150
175
200
225
250
Output Current (mA)
Output Current (mA)
SS6638-33G Load Regulation (L=47µH, CD54)
Fig. 29
SS6638-33G Efficiency (L=47µH,CD54)
3.50
1.0
3.45
Output Voltage Vout (V)
Start up
0.9
Input Voltage (V)
175
45
1.1
0.8
0.7
0.6
Hold on
0.5
0.4
0.3
0.2
0.1
0.0
150
75
50
2.6
2.4
125
SS6638-33G Efficiency (L=100µH, CD54)
55
VIN=1.2V
2.5
100
Output Current (mA)
Efficiency (%)
Output Voltage (V)
Fig. 26
VIN=1.5V
55
VIN=0.9V
3.40
3.35
No Load
3.30
3.25
3.20
3.15
3.10
3.05
0
Fig. 30
8/21/2005 Rev.2.3
2
4
6
8
10
12
14
16
18
20
Output Current (mA)
SS6638-33G Start-up & Hold-on Voltage (L=100µH CD54)
3.00
-40
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 31
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SS6638-33G Output Voltage vs. Temperature
9 of 19
SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
80
Maximum Duty Cycle (%)
Switching Frequency (kHz)
160
140
120
100
80
60
40
-40
0
20
40
60
80
100
Temperature (°C)
SS6638-33G Switching Frequency vs. Temperature
76
74
72
70
-40
1.8
45
1.6
40
1.4
1.2
1.0
0.8
0.6
0.4
-20
Fig. 33
Supply Current IDD1 (µA)
SW Turn ON Resistance (Ω)
Fig. 32
-20
78
0
20
40
60
80
100
Temperature (°C)
SS6638-33G Maximum Duty Cycle vs. Temperature
35
30
25
20
15
0.2
0.0
-40
-20
0
20
40
60
80
10
-40
100
Temperature (°C)
Fig. 34 SS6638-33G SW Turn ON Resistance vs. Temperature
-20
0
20
40
60
80
100
Temperature (°C)
SS6638-33G Supply Current vs. Temperature
Fig. 35
90
4.6
4.4
85
4.2
VIN=3.0V
Efficiency (%)
Output Voltage (V)
80
4.0
3.8
VIN=1.5V
3.6
VIN=2.0V
3.4
3.2
VIN=0.9V
VIN=1.2V
3.0
75
70
VIN=3.0V
65
VIN=2.0V
60
VIN=1.5V
2.8
VIN=0.9V
55
2.6
2.4
VIN=1.2V
50
2.2
0
50
Fig. 36
8/21/2005 Rev.2.3
100
150
200
250
300
350
Output Current (mA)
SS6638-45G Load Regulation (L=100µH)
400
0
50
100
Fig. 37
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150
200
250
300
350
400
Output Current (mA)
SS6638-45G Efficiency (L=100µH)
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SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
1.6
4.6
4.4
1.4
1.2
4.0
Input Voltage (V)
Output Voltage (V)
4.2
VIN=3.0V
3.8
VIN=1.5V
3.6
VIN=2.0V
3.4
3.2
VIN=1.2V
VIN=0.9V
3.0
Start up
1.0
Hold on
0.8
0.6
0.4
2.8
2.6
0.2
2.4
0.0
2.2
0
50
100
150
200
250
300
350
0
400
Output Current (mA)
SS6638-45G Load Regulation (L=100µH)
Fig. 38
5
Fig. 39
5.0
90
4.9
80
10
15
20
Output Current (mA)
SS6638-45G Start-up & Hold-On Voltage (L=100µH)
Supply Current (µA)
Output Voltage (V)
4.8
4.7
4.6
No Load
4.5
4.4
4.3
70
60
50
40
30
4.2
20
4.1
4.0
-40
Fig. 40
-20
0
20
40
60
80
10
100
-40
Temperature (°C)
SS6638-45G Output Voltage vs. Temperature
-20
Fig. 41
0
20
40
60
80
100
Temperature (°C)
SS6638-45G Supply Current vs. Temperature
80
Maximum Duty Cycle (%)
Switching Frequency (kHz)
160
140
120
100
80
60
40
-40
Fig. 42
8/21/2005 Rev.2.3
-20
0
20
40
60
80
100
Temperature (°C)
SS6638-45G Switching Frequency vs. Temperature
78
76
74
72
70
-40
Fig. 43
-20
0
20
40
60
80
100
Temperature (°C)
SS6638-45G Maximum Duty Cycle vs. Temperature
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SS6638G
1.8
5.5
1.6
5.0
1.4
4.5
Output Voltage (V)
SW Turn ON Resistance (Ω)
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
1.2
1.0
0.8
0.6
VIN=3.0V
VIN=2.0V
4.0
3.5
VIN=1.5V
3.0
VIN=1.2V
2.5
0.4
VIN=0.9V
2.0
0.2
0.0
1.5
-40
-20
0
20
40
60
80
0
100
Temperature (°C)
Fig. 44 SS6638-45G SW Turn ON Resistance vs. Temperature
5.5
90
5.0
Output Voltage (V)
Efficiency (%)
70
VIN=3.0V
VIN=2.0V
60
VIN=0.9V
VIN=1.5V
50
VIN=1.2V
40
100
150
200
250
300
350
400
Output Current (mA)
SS6638-50G Load Regulation ( L=100µH CD54)
Fig. 45
100
80
50
4.5
VIN=3.0V
VIN=2.0V
4.0
3.5
VIN=1.5V
3.0
VIN=1.2V
2.5
2.0
VIN=0.9V
30
20
0
50
100
150
200
250
300
350
1.5
0
400
Fig. 46
50
100
150
200
250
300
350
400
Output Current (mA)
Output Current (mA)
SS6638-50G Efficiency (L=100µH CD54)
Fig. 47
SS6638-50G Load Regulation (L=47µH CD54)
90
1.8
85
1.6
80
Input Voltage (V)
Efficiency (%)
1.4
75
70
VIN=3.0V
65
60
VIN=2.0V
55
Start up
0.8
0.6
0.4
VIN=1.2V
50
45
0
1.0
Hold on
VIN=1.5V
VIN=0.9V
1.2
0.2
0.0
50
100
150
200
250
300
350
400
0
2
Output Current (mA)
Fig. 48
8/21/2005 Rev.2.3
SS6638-50G Efficiency (L=47µH CD54)
4
6
8
10
12
14
16
18
20
Output Current (mA)
Fig. 49 SS6638-50G Start-up & Hold-on Voltage (L=100µH CD50)
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12 of 19
SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
5.3
160
Switching Frequency (kHz)
Output Voltage VOUT (V)
5.2
5.1
No Load
5.0
4.9
4.8
4.7
4.6
4.5
4.4
-40
-20
0
20
40
60
80
120
100
80
60
40
-40
100
Temperature (°C)
Fig. 50 SS6638-50G Output Voltage vs. Temperature
Fig. 51
80
-20
0
20
40
60
80
100
Temperature (°C)
SS6638-50G Switching Frequency vs. Temperature
SW Turn ON Resistance (Ω)
1.8
78
Maximum Duty Cycle (%)
140
76
74
72
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
70
-40
0.0
-20
Fig. 52
0
20
40
60
80
100
Temperature (°C)
SS6638-50G Maximum Duty Cycle vs. Temperature
-40
-20
0
20
40
60
80
100
Temperature (°C)
Fig. 53 SS6638-50G SW Turn ON Resistance vs. Temperature
100
90
VOUT
Supply Current IDD1 (µA)
80
50mV/div
70
60
50
100mA
40
Load Step
30
50mA/div
20
10
-40
-20
Fig. 54
8/21/2005 Rev.2.3
0
20
40
60
80
100
Temperature (°C)
SS6638-50G Supply Current vs. Temperature
www.SiliconStandard.com
Fig. 55 Load Transient Response
(L1=100µH, C2=47µF, VIN=2V)
13 of 19
SS6638G
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
VOUT
20mv/div
VIN
0.5V/div
Fig. 56
Line Transient Response
(L1=100µH, C2=47µF)
BLOCK DIAGRAM
SW
1.25V REF.
VOUT
1M
+
Enable
GND
OSC, 100KHz
PIN DESCRIPTIONS
PIN 1 : GND - Ground. Must be low impedance;
solder directly to ground plane.
PIN 3 : SW - Internal drain of N-MOSFET
switch.
PIN 2 : VOUT - IC supply pin. Connect VOUT to
the converter output.
8/21/2005 Rev.2.3
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14 of 19
SS6638G
APPLICATION INFORMATION
GENERAL DESCRIPTION
The SS6638G PFM (pulse frequency modulation)
converter IC combines a switch mode converter,
N-channel power MOSFET, precision voltage
reference, and voltage detector in a single
monolithic device. It offers both extreme low
quiescent current, high efficiency, and very low
gate threshold voltage to ensure start-up with low
battery voltage (0.8V typ.). Designed to maximize
battery life in portable products, it minimizes
switching losses by only switching as needed
to service the load.
of each cycle. Depending on the circuit, PFM
converters operate in either discontinuous mode
or continuous conduction mode. Continuous
conduction mode means that the inductor current
does not ramp to zero during each cycle.
VIN
IIN
ID
When the output voltage drops, the error
comparator enables the 100KHz oscillator that turns
the MOSFET on for about 7.5µs and off for 2.5µs.
Turning on the MOSFET allows inductor current to
ramp up, storing energy in a magnetic field. When
The MOSFET turns off, inductor current is forced
through the diode to the output capacitor and load. As
the stored energy is depleted, the current ramps
down until the diode turns off. At this point,
the inductor may ring due to residual energy and stray
capacitance. The output capacitor stores charge when
the current flowing through the diode is high, and
releases it when current is low, thereby maintaining
a steady voltage across the load.
As the load increases, the output capacitor
discharges faster and the error comparator initiates
cycles sooner, increasing the switching frequency.
The maximum duty cycle ensures adequate time for
energy transfer to the output during the second half
8/21/2005 Rev.2.3
VOUT
+
EXT
PFM converters transfer a discrete amount of
energy per cycle and regulate the output voltage
by modulating the switching frequency with a
constant pulse width. Switching frequency
depends on load, input voltage, and inductor
value, and it can range up to 100kHz. The SW
on-resistance is typically 1 to 1.5Ω to minimize
switching losses.
IOUT
SW
Isw
Ico
SS6638G
VEXT
IIN
IPK
ISW
Charge Co.
ID
VSW
IOUT
TDIS
Discharge Co.
t
Discontinuous Conduction Mode
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15 of 19
SS6638G
VEXT
fsw =
IIN
IPK
(VOUT + VD − VIN)
×
TON (VOUT + VD − VSW )
VIN − VSW
x
)]
[1 + (
2 VOUT + VD − VSW
≅
IV
1
1  VOUT + VD − VIN 


TON  VOUT + VD − VSW 
where Vsw = switch drop and is proportional
to output current.
ISW
ID
INDUCTOR SELECTION
IOUT
VSW
t
Continuous Conduction Mode
At the boundary between continuous and
discontinuous mode, output current (IOB) is
determined by
VIN

 1 VIN
TON(1 − x )
IOB = 

 VOUT + VD  2 L
where VD is the diode drop,
x = (RON+Rs)Ton/L.
RON= Switch turn on resistance, Rs= Inductor DC
resistance
TON = Switch ON time
In the discontinuous
frequency (Fsw) is
Fsw =
mode,
the
switching
2(L)(VOUT + VD − VIN)(IOUT)
(1 + x )
VIN 2 × TON 2
In the continuous mode, the switching frequency
is
To operate as an efficient energy transfer
element, the inductor must fulfill three
requirements. First, the inductance must be low
enough for the inductor to store adequate energy
under the worst-case condition of minimum input
voltage and switch ON time. Second, the
inductance must also be high enough so the
maximum current rating of the SS6638 and inductor
are not exceeded at the other worst-case condition
of maximum input voltage and ON time. Lastly,
the inductor must have sufficiently low DC
resistance so excessive power is not lost as heat
in the windings. Unfortunately this is inversely
related to physical size.
Minimum and maximum input voltage, output
voltage and output current must be established
before an inductor can be selected.
In discontinuous mode operation, at the end of
the switch ON time, peak current and energy in
the inductor build according to
Ron + Rs
 Vin  

Ton) 
IPK = 
 1 − exp( −
L
 Ron + Rs  

x
 VIN 

≅
 (TON) 1 − 
2
 L 

≅
VIN
TON
L
(Simple losses equation),
where x=(RON+RS)TON/L
8/21/2005 Rev.2.3
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16 of 19
SS6638G
EL =
 VOUT + VD − VSW x 
IPK = 
−  IOUT +
VIN − VSW
2

x

 VIN − VSW 

 TON 1 − 
2L
2



1
L × IPK 2
2
Power required from the inductor per cycle must
be equal or greater than
PL/FSW = (VOUT + VD − VIN)(IOUT)(
1
FSW
Valley current (Iv) is
)
 VIN − VSW 
 VOUT + VD − VSW x 
Iv = 
−  IOUT − 
×
VIN − VSW
2
2L



x

TON 1 − 
2


in order for the converter to regulate the output.
When loading is over IOB, PFM converter
operates in continuous mode. Inductor peak
current can be derived from
Table 1 Indicates resistance and height for each coil.
Inductance
Power Inductor Type
( µH )
Coilcraft SMT Type
(www.coilcraft.com)
Resistance
(Ω)
Rated Current
(A)
22
0.10
0.7
47
0.18
0.5
100
0.38
0.3
22
0.08
2.7
47
0.14
1.8
47
0.25
0.7
100
0.50
0.5
DS1608
DO3316
Sumida SMT Type CD54
Hold
SMT Type PM54
Hold
SMT Type PM75
47
0.25
0.7
100
0.50
0.5
33
0.11
1.2
CAPACITOR SELECTION
A poor choice for an output capacitor can result in
poor efficiency and high output ripple. Ordinary
aluminum electrolytics, while inexpensive, may
have unacceptably poor ESR and ESL. There are
low ESR aluminum capacitors for switch mode
DC-DC converters which work much better than
general proposetypes. Tantalum capacitors
provide still better performance but are more
expensive. OS-CON capacitors have extremely
low ESR in a small size. If capacitance is
reduced, output ripple will increase.
Height
(mm)
2.9
5.2
4.5
4.5
5.0
should be at least 1.25 times greater than the
maximum input voltage.
DIODE SELECTION
Speed, forward drop, and leakage current are the
three main considerations in selecting a rectifier
diode. Best performance is obtained with
a Schottky rectifier diode, such as the 1N5818,
or the SS13 and B0530W in surface mount packages.
For lower output power a 1N4148 can be used
although efficiency and start-up voltage will suffer
substantially.
Most of the input supply is provided by the input
bypass capacitor; the capacitor voltage rating
8/21/2005 Rev.2.3
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17 of 19
SS6638G
COMPONENT POWER DISSIPATION
VD = Diode drop.
Operating in discontinuous mode, power loss in
the winding resistance of inductor can be
approximated to
The power dissipated due to the switch loss is
PD L =
PDsw =
2  TON 
 VOUT + VD 
 (POUT )

 (Rs ) 
3 L 
 VOUT 
2  TON 
 VOUT + VD − VIN 
 (POUT )

 (RON) 
3 L 
VOUT


The power dissipated in the rectifier diode is
 VD 
PDD = 
 (POUT)
 VOUT 
where POUT=VOUT ×IOUT ; Rs=Inductor DC R;
PHYSICAL DIMENSIONS (unit: mm)
SOT-23-3 (GU)
D
0.25
E1
E
c
L
e
θ
L1
e1
A2
A
A1
b
SYMBOL
MIN
MAX
A
0.95
1.45
A1
0.05
0.15
A2
0.90
1.30
b
0.30
0.50
c
0.08
0.22
D
2.80
3.00
E
2.60
3.00
E1
1.50
1.70
e
0.95 BSC
e1
1.90 BSC
L
0.30
L1
θ
8/21/2005 Rev.2.3
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0.60
0.60 REF
0˚
8˚
18 of 19
SS6638G
PHYSICAL DIMENSIONS (unit: mm) (Continued)
SOT-89-3
(GX)
D
A
SYMBOL
MIN
MAX
C
A
1.40
1.60
B
0.44
0.56
B1
0.36
0.48
C
0.35
0.44
D
4.40
4.60
D1
1.50
1.83
E
2.29
2.60
D1
H
E
L
B
e
B1
e1
e
1.50 BSC
e1
3.00 BSC
H
3.94
4.25
L
0.89
1.20
TO-92 (GZ)
SYMBOL
MIN
MAX
A
4.32
5.33
b
0.36
0.47
D
4.45
5.20
E
3.18
4.19
e
2.42
2.66
e1
1.15
1.39
j
3.43
-
L
12.70
-
S
2.03
2.66
A
D
b
S
E
L
j
e1
e
Information furnished by Silicon Standard Corporation is believed to be accurate and reliable. However, Silicon Standard Corporation makes no
guarantee or warranty, express or implied, as to the reliability, accuracy, timeliness or completeness of such information and assumes no
responsibility for its use, or for infringement of any patent or other intellectual property rights of third parties that may result from its
use. Silicon Standard reserves the right to make changes as it deems necessary to any products described herein for any reason, including
without limitation enhancement in reliability, functionality or design. No license is granted, whether expressly or by implication, in relation to
the use of any products described herein or to the use of any information provided herein, under any patent or other intellectual property rights of
Silicon Standard Corporation or any third parties.
8/21/2005 Rev.2.3
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