AN2627 Application note
ST8R00 syncronous boost conver ter with output current cut-off function
Introduction
The ST8R00 family of synchronous step-up DC-DC converters with current output cut-off function provide up to 1 A over an input voltage range of 4 V to 6 V and an output voltage range of 6 V to 12 V. The high switching frequency (1.2 MHz) allows the use of tiny surface-mount components. Along with the resistor divider to set the output voltage value, an inductor and two capacitors are required. A low output ripple is guaranteed by the current mode PWM topology and by the use of low ESR surface-mounted ceramic capacitors. The device is available in two versions: burst mode (ST8R00) and continuous mode (ST8R00W). The ST8R00 devices are thermal protected and available in the DFN8 4x4 package. Figure 1. Simplified schematic diagram
LX Thermal Ps Ns INH Inhibit PGND Po OUT
PWM control
FB IN Vref
PGND
GND
AM00001v1
May 2008
Rev 1
1/18
www.st.com
Contents
AN2627
Contents
1 ST8R00 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2
Selecting components for applications . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 2.2 2.3 2.4 2.5 Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 4
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Demonstration board usage recommendation . . . . . . . . . . . . . . . . . . . 13
4.1 External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.1 4.1.2 Capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5 6 7
BOM with most-used components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Footprint recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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List of figures
List of figures
Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Simplified schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 ST8R00 inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 ST8R00W inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ST8R00W inductor current at no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ST8R00 cut-off block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Current cut-off function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 ST8R00 application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Inhibit voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Voltage feedback vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 The ST8R00 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Demonstration board layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Efficiency vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 ST8R00 efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 ST8R00W efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 DFN8 4x4 recommended footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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ST8R00 description
AN2627
1
ST8R00 description
The ST8R00 is a family of adjustable current mode PWM synchronous step-up DC-DC converters with internal 1 A power switch. It represents a complete 1 A switching regulator with internal compensation which eliminates the need for additional components. The two devices in the family, the ST8R00 and ST8R00W, operate at light load in two different ways. The ST8R00 works in power-save mode to achieve good efficiency, as shown in Figure 2. The ST8R00W, in order to guarantee the lowest switching ripple, operates in PWM (pulse width modulation) mode as show in Figure 3 and Figure 4. At medium and high load current, both versions operate in PWM mode. The thermal shutdown block turns off the regulator when the junction temperature exceeds 150 C (typ), and the cycle-by-cycle current limiting provides protection against overcurrent sink. Figure 2. ST8R00 inductor current at light load
IL
LX
VIN=5 V, VOUT=8 V, IOUT=60 mA, Ch1=LX, Ch4=IL
4/18
AN2627 Figure 3. ST8R00W inductor current at light load
ST8R00 description
IL
LX
VIN=5 V, VOUT=8 V, IOUT=60 mA, Ch1=LX, Ch4=IL Figure 4. ST8R00W inductor current at no load
IL
LX
VIN=5 V, VOUT=8 V, no load Ch1=LX, Ch4=IL For proper functioning of the device, only a few components are required: an inductor, two capacitors and the resistor divider. The inductor chosen must not saturate at the operating peak current. Its value should be selected taking into account that a large inductor value reduces output voltage ripple, while a smaller inductor can be selected when it is important to reduce package size and the total cost of the application. Finally, the ST8R00 family has been designed to work properly with X5R or X7R SMD ceramic capacitors both at the input and at the output. These types of capacitors, thanks to their very low series resistance (ESR), minimize the output voltage ripple. Other low ESR capacitors can be used in accordance with application requirements without compromising the correct functionality of the device. This device features an output current cut-off function. Two P-channel MOSFETs in a backto-back configuration, as shown in Figure 5, stop the output current when the inhibit is low (Figure 6).
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ST8R00 description Figure 5. ST8R00 cut-off block
LX OUT Ps Ns PGND Po
AN2627
Figure 6.
Current cut-off function
Iout Vout Inh
Vin
Figure 7 shows the in-rush current at start-up. Initially, the COUT capacitor is completely discharged and the current limitation is due only to the equivalent series resistor of the inductor, the power MOSFET parasitic diode and the cut-off MOSFETs' RDS(ON). As soon as the output voltage reaches the input voltage level, the device begins to switch and the current is limited cycle by cycle.
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AN2627 Figure 7. Inrush current
ST8R00 description
Vout
Iin
LX
VIN=4.5 V, VOUT=7 V, VINH from 0 V to 3 V, RLOAD=13 , L=10 H, CIN=COUT=10 F
1.1
Inhibit function
The ST8R00 family of devices also include an inhibit function (pin 6). When the INH voltage is higher than 2 V, the device is ON and if it is lower than 0.8 V, the device is OFF. The INH pin does not have an internal pull-up, which means that the pin cannot be left floating. If the inhibit function is not used, the INH pin must be connected to VIN as in the schematic in Figure 8 below. Figure 8. ST8R00 application schematic
Vin Rinh
6 4
L
1
IN INH
Cin Cinh
7
LX OUT
8
ST8R00
FB PGND
2
5
HV GND
3
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ST8R00 description Figure 9. Inhibit voltage vs. temperature
AN2627
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 - 75
Vinh [V]
Vin=4V, Vinh from 0 to 2V, Iout=50mA, L=4.7H, Cin=10F, Cout=10F
ON
OFF
- 50
- 25
0
25
50 T [C]
75
100 125 150 175
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AN2627
Selecting components for applications
2
Selecting components for applications
This section provides information to assist in the selection of the most appropriate components for applications. Figure 10 shows a typical application schematic diagram. Figure 10. Typical application schematic
Vin
4
L
1
IN C i n O F F ON
6
INH
7
LX OUT
8
Vout R1
ST8R00
FB PGND
2
5
HV GND
3
Cout R2
2.1
Output voltage selection
The output voltage can be adjusted from 6 V up to 12 V by connecting a resistor divider between the output and the FB pin. The resistor divider should be chosen in accordance with the following equation: Equation 1
R1 V o u t = V F B 1 + -----R2 with V F B = 1.22 V
The feedback voltage versus temperature is shown Figure 11 below. It is recommended to use a resistor with a value in the range of 10 k to 100 k . Lower values can be suitable as well, but will increase current consumption. Figure 11. Voltage feedback vs. temperature
1.3 1.28 1.26 1.24 1.22 1.2 1.18 1.16 1.14 1.12 1.1 -75 -50
Vfb [mV]
Vin=Vinh=5V, Iout=50mA, L=4.7H, Cin=10F, Cout=10F
-25
0
25
50
75
100
125
150
175
T [C]
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Selecting components for applications
AN2627
2.2
Input capacitor
The input capacitor must be able to provide AC ripple current to the inductor and to withstand the maximum input operating voltage. Another important function of the input capacitor is to limit noise and therefore the interference with the other blocks connected to the same network. The quality of these capacitors must to be quite high to minimize the power dissipation generated by the internal ESR, thereby improving system reliability and efficiency. Various capacitors can be considered:
Ceramic capacitors - These capacitors usually have a higher RMS current rating for a given physical dimension (due to the very low ESR). The drawback is the high cost of capacitors with very large values. Electrolytic capacitor - The availability of small size tantalum capacitors with very low ESR is increasing. However, they are subject to thermal damage if subjected to very high current during charge. Since they can, in fact, be subjected to high surge current when connected to the power supply, it is better to avoid using this type of capacitor for the input filter of the device. Aluminum capacitors are not the best choice due to their high ESR.
2.3
Inductor
The inductor value is very important because it establishes the ripple current. The approximate inductor value is obtained with the following formula: Equation 2
Vi n L = ------- T o n I L
where TON is the ON time of the internal switch, given by D · TSW. The ripple current, IL, is usually fixed at 20-40% of IIN_MAX. Equation 3
Ii n
max
Io u t m a x Vo u t = -----------------------------------Vi n
where is the efficiency.
2.4
Output capacitor
The output capacitor is very important to satisfy the output voltage ripple requirement. To reduce the output voltage ripple, a low ESR capacitor is required. The output voltage ripple (VRIPPLE), in continuous mode is: Equation 4
( Vo u t V n ) V R I P P L E = I o u t E S R + -------------------------------i------------ V C F
out out SW
where FSW is the switching frequency.
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AN2627
Selecting components for applications
2.5
Layout considerations
Due to the high switching frequency and peak current, the layout is an important design step for all switching power supplies. If the layout is not done carefully, important parameters such as efficiency and output voltage ripple could be out of specification. Shor t, wide traces must be implemented for main current and for power ground paths as shown in bold in Figure 12. The input capacitor must be placed as close as possible to the IC pins as well as the inductor and output capacitor. A common ground node minimizes ground noise, as shown in Figure 12. The HV pin must be floating or connected to GND and the exposed pad of the package must be connected to the common ground node. Figure 12. Layout considerations
Vin
4
L
1
IN Cin O F F ON
6
INH
7
LX OUT
8
Vout R1 R2 Cout
ST8R00
FB PGND
2 3
5
HV GND
11/18
Thermal considerations
AN2627
3
Thermal considerations
The dissipated power of the device is related to three different sources:
Switching losses due to the (not negligible) RDS(ON). These are equal to:
Equation 5
PO N
N
= RD S ( o n )
N
I
2
out
D
and Equation 6
PO N
P
= RD S ( o n )
PEQ
I
2
out
1D
where D is the duty cycle of the application and RDS(ON)_PEQ=RDS(ON)_PS+RDS(ON)_PO. Note: the duty cycle is theoretically given by:
V in 1 -----------V out
but in practice it is quite higher than this value to compensate for the losses of the overall application. For this reason, the switching losses related to the RDS(ON) increase compared to an ideal case.
Switching losses due to its turning on and off. These are calculated using the following equation:
Equation 7
( To n + To f f ) P S W = V i n I o u t ----------------------------- F S W = V i n I o u t T S W F S W 2
where TON and TOFF are the overlap times of the voltage across the power switch and the current flowing into it during the turn-on and turn-off phases. TSW is the equivalent switching time.
Quiescent current losses:
Equation 8
PQ = Vi n IQ
where IQ is the quiescent current. The overall losses are: Equation 9
PT O T = RD S ( o n )
N
I
2
out
D + RD S ( o n )
PEQ
I
2
out
( 1 D ) + Vi n Io u t TS W FS W + Vi n IQ
The junction temperature of device will be: Equation 10
TJ = TA + R t hJ A PT O T
where TA is the ambient temperature and RthJ-A is the thermal resistance junction-toambient.
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AN2627
Demonstration board usage recommendation
4
Demonstration board usage recommendation
The demonstration board shown in Figure 13 is provided with a Kelvin connection, so for each pin there are two lines available: one used to supply or sink current, and the other used to perform the needed measurement. The ST8R00 inhibit pin does not have an internal pull-up, so the inhibit pin cannot be left floating. Figure 13. The ST8R00 demonstration board
Figure 14. Demonstration board layers
Top layer
Bottom layer
The board has one inhibit pin available, located on the top left of the board. The inhibit pin can be used to supply an external voltage higher than 2 V to turn on the device, or an external voltage lower than 0.8 V to turn off the device.
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Demonstration board usage recommendation
AN2627
4.1
External component selection
Figure 15 shows the schematic diagram of the demonstration board. Figure 15. Demonstration board schematic
Vin
4
L
1
IN C i n O F F ON
6
INH
7
LX OUT
8
Vout R1
ST8R00
FB PGND
2
5
HV GND
3
Cout R2
In order to obtain the needed output voltage, the resistor divider must be selected based on the following formula: Equation 11
R1 V o u t = V F B 1 + ------R2
with
V F B = 1.22 V
Table 1.
Recommended resistor divider
Vout 8V 9.5 V R1 56 k 68 k R2 10 k 10 k
The resistors in Table 1 represent a good compromise in terms of current consumption and minimum output voltage.
4.1.1
Capacitor selection
It is possible to use any X5R or X7R ceramic capacitor:
CIN=10 F (ceramic) or higher COUT=10 F (ceramic) or higher. It is possible to put several capacitors in parallel to reduce the equivalent series resistance and improve the ripple present in the output voltage.
4.1.2
Inductor selection
Due to the high (1.2 MHz) frequency, it is possible to use very small inductor values. In the demonstration board, the device was tested with inductors in the range of 1 H to 10 H, with very good efficiency performance (see Figure 18 and Figure 19). Because the device is able to provide an operating output current of 1 A, we strongly recommend the use of inductors capable of managing at least 3.5 A.
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AN2627
Demonstration board usage recommendation Figure 16. Efficiency vs. output current
100 90
Efficiency [%]
80 70 60 50 40 30 20 0 0.1 0.2 0.3 0.4 0.5 0.6 Iout [A] 0.7 0.8 0.9 1
Vin=5V , Vinh=5V, L=4.7H, Cin=10F, Cout=10F, Vout=8V
ST 8R00 ST 8R00W
Figure 17. Efficiency vs. output voltage
100 95 90
Efficiency [%]
85 80 75 70 65 60 55 50 4 .5 5 .5 6 .5 7 .5 8 .5 9 .5 1 0 .5 1 1 .5 1 2 .5
Vin=5V , Vinh=5V, L=4.7H, Cin=10F, Cout=10F, Iout=300mA
ST 8R00 ST 8R00W
Vout [V]
Figure 18. ST8R00 efficiency vs. inductor
100 90
Efficiency [%]
80 70 60 50 40 30 20 0 2 4 6 L [H] 8 10
Vin=5V , Vinh=5V, Vout=8V, Cin=10F, Cout=10F
Iou t=300mA Iou t=0.5A Iou t=1A
15/18
BOM with most-used components Figure 19. ST8R00W efficiency vs. inductor
100 90
Efficiency [%]
AN2627
80 70 60 50 40 30 20 0 2 4 6 L [H] 8 10
Vin=5V , Vinh=5V, Vout=8V, Cin=10F, Cout=10F
Iou t=300mA Iou t=0.5A Iou t=1A
5
BOM with most-used components
Table 2.
Name CIN COUT L
Bill of materials
Value 10 F 10 F 4.7 H Material Ceramic Ceramic Manufacturer Murata Murata Coiltronics Part numbers GRM31CR61E106KA12B GRM31CR61E106KA12B DR73-4R7
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AN2627
Footprint recommended data
6
Footprint recommended data
Figure 20. DFN8 4x4 recommended footprint
7
Revision history
Table 3.
Date 13-May-2007
Document revision history
Revision 1 Initial release Changes
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AN2627
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