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AN2373 Application Note
STw4141 dual output DC to DC converter with resistor dividers in feedback loops
Introduction
The proposal of this application note is to give user instruction and information about modification of feedback loops in order to change output voltages.
June 2006
Rev 1
1/15
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Contents
AN2373
Contents
1 Application schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 1.2 Modified application schematic with two resistor dividers . . . . . . . . . . . . . 3 Modified application schematic with one resistor divider . . . . . . . . . . . . . . 4
2 3
Calculating resistor values in divider . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Recommended resistor dividers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. 1 3. 2 3. 3 3. 4 STA1 (1.8V/1.2V) voltage option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 STA2 (1.8V/1.3V) voltage option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 STA3 (1.8V/1.4V) voltage option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 STA4 (1.8V/1.5V) voltage option . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4 5 6
Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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AN2373
Application schematic description
1
Application schematic description
The Dual output DC-DC step down converters shown in Figure 1: Dual DC-DC converter schematic with resistor dividers in both feedback loops1 and Figure 2: Dual DC-DC converter schematic with resistor divider in voltage1 feedback loop are based on STw4141 device with additional resistor dividers either in both or in one feedback loop. This modification permits to achieve different output voltage combinations than preset fixed output voltages. Condition VOUT2 < V O U T 1 3.3 V must still be kept due to the device architecture and output voltages limit. In order to maintain the good feedback loop performance, the total value of each parallel combination R1.1 | | R R 1.2 and R 2.1 | | R 2.2 must be equal or less than 10 k .
1.1
Figure 1.
Modified application schematic with two resistor dividers
Dual DC-DC converter schematic with resistor dividers in both feedback loops
L 4.7 H VLX1 VLX2
A3 A4
VIN=3.6V to 5.5V C1 10 F 6.3 V
PVDD
B1
A2
VOUT1
VOUT1=3.3V C1.1 22 F 6.3 V
VDD
D3 D1
R1.1 FB1 15k R1.2 18k
EN
D4
AUTO
C2
STw4141
STA1
B4
VOUT2
VOUT2=1.8V C2.1 22 F 6.3 V
R2.1 VSEL
C3 D2
FB2
10k R2.2 20k
MODE/SYNC
C1 B2 A1 C4 B3
STATE
PGND GND
T_MODE
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Application schematic description
AN2373
1.2
Figure 2.
Modified application schematic with one resistor divider
Dual DC-DC converter schematic with resistor divider in voltage1 feedback loop
L 4.7 H VLX1 VLX2
A3 A4
VIN=3.1V to 5.5V C1 10 F 6.3 V
PVDD
B1
A2
VOUT1
VOUT1=2.8V C1.1 22 F 6.3 V
VDD
D3 D1
R1.1 FB1 15k R1.2 27k
EN
D4
AUTO
C2
STw4141
STA4
B4
VOUT2
VOUT2=1.5V C2.1 22 F 6.3 V
VSEL
C3
D2
FB2
MODE/SYNC
C1 B2 A1 C4 B3
STATE
PGND GND
T_MODE
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AN2373
Calculating resistor values in divider
2
Calculating resistor values in divider
This section lists the equations to use in order to calculate the resistor values in divider. 1. 2. . Choose R1.2 higher than 10 k Calculate R1.1 by equation (1).
R1.1 =
3.
(VOUT 1NEW - VOUT 1) R1.2 VOUT 1
[]
(1)
Check if condition (2) is valid.
5 k
4. 5.
R1.1 R1.2 10 k R1.1 + R1.2
(2)
Choose R2.2 higher than 10 k . Calculate R2.1 by equation (3).
R2.1 =
6.
(VOUT 2 NEW - VOUT 2) R2.2 VOUT 2 R2.1 R2.2 10 k R2.1 + R2.2
...original output voltages
[]
(3)
Check if condition (4) is valid..
5 k
VOUT1, VOUT2
(4)
VOUT1NEW, VOUT2NEW ...requested output voltages
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Recommended resistor dividers
AN2373
3
3.1
Recommended resistor dividers
STA1 (1.8V/1.2V) voltage option
Table 1.
VOUT1 [V]
Recommended resistor divider for STA1 (1.8V/1.2V) voltage option
VOUT2 [V] 2.5 1.9 R1.1 [k] R1.2 [k] R2.1 [k] 12 13 15 1.8 1.2 2.5 1.9 18 10 20 not used(1) 12 13 15 1.8 1.2 2.5 1.9 27 10 20 not used(1) 12 13 12 1.8 1.2 1.9 24 10 20 not used(1) 13 13 33 10 not 22 20 used(1) 2.8 to 5.5 11 22 3 to 5.5 11 22 3.1 to 5.5 R2.2 [k] 11 22 3.6 to 5.5 VIN range [V]
3.3
2.8
2.7
2.5
1.8 1.2
1. Only one resistor divider in output1 feedback loop is used (as it is shown in Figure 2)
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AN2373
Recommended resistor dividers
3.2
STA2 (1.8V/1.3V) voltage option
Table 2.
VOUT1 [V]
Recommended resistor dividers for STA2 (1.8V/1.3V) voltage option
VOUT2 [V] 2.5 1.9 R1.1 [k] R1.2 [k] R2.1 [k] 12 11 15 1.8 1.3 2.5 1.9 18 9.1 24 not used(1) 12 11 15 1.8 1.3 2.5 1.9 27 9.1 24 not used(1) 12 11 12 1.8 1.3 1.9 24 9.1 24 not used(1) 11 13 33 9.1 not 24 24 used(1) 2.8 to 5.5 13 24 3 to 5.5 13 24 3.1 to 5.5 R2.2 [k] 13 24 3.6 to 5.5 VIN range [V]
3.3
2.8
2.7
2.5
1.8 1.3
1. Only one resistor divider in output1 feedback loop is used (as it is shown in Figure 2)
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Recommended resistor dividers
AN2373
3.3
STA3 (1.8V/1.4V) voltage option
Table 3.
VOUT1 [V]
Recommended resistor dividers for STA3 (1.8V/1.4V) voltage option
VOUT2 [V] 2.5 1.9 R1.1 [k] R1.2 [k] R2.1 [k] 12 12 15 1.8 1.4 2.5 1.9 18 11 39 not used(1) 12 12 15 1.8 1.4 2.5 1.9 27 11 39 not used(1) 12 12 12 1.8 1.4 1.9 24 11 39 not used(1) 12 13 33 11 not 33 39 used(1) 2.8 to 5.5 15 33 3 to 5.5 15 33 3.1 to 5.5 R2.2 [k] 15 33 3.6 to 5.5 VIN range [V]
3.3
2.8
2.7
2.5
1.8 1.4
1. Only one resistor divider in output1 feedback loop is used (as it is shown in Figure 2)
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AN2373
Recommended resistor dividers
3.4
STA4 (1.8V/1.5V) voltage option
Table 4.
VOUT1 [V]
Recommended resistor dividers for STA4 (1.8V/1.5V) voltage option
VOUT2 [V] 2.5 1.9 R1.1 [k] R1.2 [k] R2.1 [k] 10 9.1 15 1.8 1.5 2.5 1.9 18 10 51 not used(1) 10 9.1 15 1.8 1.5 2.5 1.9 27 10 51 not used(1) 10 9.1 12 1.8 1.5 1.9 24 10 51 not used(1) 10 13 33 9.1 not 15 33 used(1) 2.8 to 5.5 15 33 3 to 5.5 15 33 3.1 to 5.5 R2.2 [k] 15 33 3.6 to 5.5 VIN range [V]
3.3
2.8
2.7
2.5
1.8 1.5
1. Only one resistor divider in output1 feedback loop is used (as it is shown in Figure 2)
Resistors R1.1, R1.2, R2.1, R2.2 have to be at least 1% or less tolerance. All recommended values of resistors are chosen from E24 series. Since current flowing through the dividers is very low, case size 0402, 0201 or less could be used.
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Measurements
AN2373
4
Measurements
STw4141 STA1 device with added resistor dividers (output voltages 3.3V/1.8V) has been tested in order to check the application functionality. Some basic tests like static parameters measurement, load transient response in PWM mode were done.
Static measurement
VBAT = 5 V, EN = `1', VSEL = `1', MODE/SYNC = `1', TM = `0', AUTO = `0', TA = 25 C. Figure 3. Output1 voltage versus output currents
3.4
3.35
3.3
VOUT1 (V)
3.25
200 100 25 12.5 12.5 200 400 50 50 100
3.35-3.4 3.3-3.35 3.25-3.3 3.2-3.25
3.2
IOUT1 (mA)
3.1 1.6 0.8 0.8 3.1 1.6 6.3
25
6. 3
IOUT2 (mA)
Figure 4.
Output 2 voltage versus output currents
1.85
1.83
1.81
VOUT2 (V)
1.79 1.83-1.85 1.81-1.83 1.79-1.81 1.77-1.79 1.75-1.77
1.77 200 100 100 25 12.5 12.5 200 400 50 50
1.75
IOUT1 (mA)
3.1 1.6 0.8 0.8 3.1 1.6 6.3
25
6.3
IOUT2 (mA)
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AN2373
Measurements
Load transients
VBAT = 5 V, EN = `1', VSEL = `1', MODE/SYNC = `1', TM = `0', AUTO = `0', current steps IOUT1 = from 1 mA to 300 mA, IOUT2 = from 1 mA to 400 mA,trise = tfall = 1 s, TA = 25 C. Figure 5. Load transient related to in-phase switching
Figure 6.
Load transient output 1 leading output 2
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Measurements Figure 7. Load transient output 2 leading output 1
AN2373
Figure 8.
Load transient related to anti-phase switching
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AN2373
Summary
5
Summary
In order to provide different output voltages of STw4141 device the resistor dividers can be added to feedback loops. Usage of resistor dividers provides more flexibility to achieve different output voltages than preset values. For example: Output1 voltage 3.3 V, Output2 voltage 1.8 V at supply voltage from 3.6 V to 5.5 V. Current consumption is increased by additional current flowing through resistor dividers. Due to step-down converter topology for V O U T 1 2.4 V input voltage must be kept in range from VOUT1+0.3 V to 5.5 V. Overall, the measurements verified good functionality of STw4141 device with resistor dividers at specified conditions.
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Revision history
AN2373
6
Revision history
Table 5.
Date 13-Jun-2006
Document revision history
Revision 1 Initial release. Changes
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AN2373
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