cd00171396

AN2627
Application note
ST8R00 syncronous boost
converter 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
OUT
Thermal
Ps
Po
Ns
INH
Inhibit
PGND
PWM control
FB
IN
Vref
PGND
GND
AM00001v1
December 2009
Doc ID 13913 Rev 2
1/19
www.st.com
Contents
AN2627
Contents
1
ST8R00 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
2
Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Selecting components for applications . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1
Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3
Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4
Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.5
Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4
Demonstration board usage recommendation . . . . . . . . . . . . . . . . . . . 13
4.1
External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.1
Capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.2
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5
BOM with most-used components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6
Footprint recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2/19
Doc ID 13913 Rev 2
AN2627
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
Doc ID 13913 Rev 2
3/19
ST8R00 description
1
AN2627
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
4/19
Doc ID 13913 Rev 2
AN2627
ST8R00 description
Figure 3.
ST8R00W inductor current at light load
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).
Doc ID 13913 Rev 2
5/19
ST8R00 description
Figure 5.
AN2627
ST8R00 cut-off block
LX
OUT
Ps
Po
Ns
PGND
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.
6/19
Doc ID 13913 Rev 2
AN2627
ST8R00 description
Figure 7.
Inrush current
Vout
Iin
LX
VIN=4.5 V, VOUT=7 V, VINH from 0 V to 3 V, RLOAD=13 Ω, L=10 µH, CIN=COUT=10 µF
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
L
Vin
4
IN
Rinh
6
Cin
Cinh
1
7
Vout
LX
8
OUT
INH
R1
ST8R00
HV
GND
PGND
3
Doc ID 13913 Rev 2
FB
Cout
5
R2
2
7/19
ST8R00 description
Vinh [V]
Figure 9.
AN2627
Inhibit voltage vs. temperature
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Vin=4V, Vinh from 0 to 2V, Iout=50mA,
L=4.7µH, Cin=10µF, Cout=10µF
-75
-50
-25
0
25
50
T [°C]
8/19
Doc ID 13913 Rev 2
ON
75
OFF
100 125 150 175
AN2627
2
Selecting components for applications
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
L
Vin
4
1
IN
OFF ON
Cin
6
7
LX
OUT
INH
ST8R00
FB
HV
GND
Cout
5
R2
2
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
R
V out = V FB 1 + ------1R2
with V FB = 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 kΩ to 100 kΩ. Lower
values can be suitable as well, but will increase current consumption.
Figure 11. Voltage feedback vs. temperature
Vfb [mV]
2.1
R1
PGND
3
Vout
8
1.3
1.28
1.26
1.24
1.22
1.2
1.18
1.16
1.14
1.12
1.1
Vin=Vinh=5V, Iout=50mA,
L=4.7µH, Cin=10µF, Cout=10µF
-75
-50
-25
0
25
50
75
100
125
150
175
T [°C]
Doc ID 13913 Rev 2
9/19
Selecting components for applications
2.2
AN2627
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:
2.3
●
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.
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
L=
Vin
⋅ TON
ΔIL
where TON is the ON time of the internal switch, given by D · TSW. The ripple current, ΔIL, is
usually fixed at 20-40% of IIN_MAX.
Equation 3
IIN_MAX =
IOUT_max ⋅ Vout
Vin ⋅ η
where η 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
( V out – V in ) ⎞
V RIPPLE = I out ⋅ ⎛ ESR + ------------------------------------------⎝
V out ⋅ C out ⋅ F SW⎠
where FSW is the switching frequency.
10/19
Doc ID 13913 Rev 2
AN2627
2.5
Selecting components for applications
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.
Short, 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
L
Vin
4
1
IN
OFF ON
6
LX
OUT
INH
Cin
7
Vout
8
ST8R00
5
R1
Cout
FB
HV
GND
3
PGND
R2
2
Doc ID 13913 Rev 2
11/19
Thermal considerations
3
AN2627
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
PON_N = RDSON_N⋅ [IOUT /(1 − D)]² ⋅ D
and
Equation 6
2
PON_P = RDSON_PEQ ⋅ IOUT
⋅ (1 − D)
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
in1 – -----------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
PSW = VIN ⋅ I OUT ⋅
(t ON + t OFF )
⋅ FSW = VIN ⋅ I OUT ⋅ t R -F ⋅ FSW
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. tR-F is the equivalent switching
time.
●
Quiescent current losses:
Equation 8
PQ = VIN ⋅ IQ
where IQ is the quiescent current.
The overall losses are:
Equation 9
2
PTOT = RDSON_N ⋅ (IOUT / 1− D)² ⋅ D + RDSON_PEQ ⋅ IOUT
⋅ (1− D) + VIN ⋅ IOUT ⋅ tR-F ⋅ FSW + VIN ⋅ IQ
The junction temperature of device will be:
Equation 10
TJ = TA + R thJA ⋅ PTOT
where TA is the ambient temperature and RthJA is the thermal resistance junction-toambient.
12/19
Doc ID 13913 Rev 2
AN2627
4
Demonstration board usage recommendation
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.
Doc ID 13913 Rev 2
13/19
Demonstration board usage recommendation
4.1
AN2627
External component selection
Figure 15 shows the schematic diagram of the demonstration board.
Figure 15. Demonstration board schematic
L
Vin
4
1
LX 8
OUT
IN
OFF ON
Cin
6
7
INH
ST8R00
FB
HV
GND
R1
5
Cout
R2
PGND
3
Vout
2
In order to obtain the needed output voltage, the resistor divider must be selected based on
the following formula:
Equation 11
V out = V FB 1 + R1
-------R2
Table 1.
with
V FB = 1.22 V
Recommended resistor divider
Vout
R1
R2
8V
56 kΩ
10 kΩ
9.5 V
68 kΩ
10 kΩ
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:
4.1.2
●
CIN=10 µF (ceramic) or higher
●
COUT=10 µF (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.
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 µH to 10 µH,
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.
14/19
Doc ID 13913 Rev 2
AN2627
Demonstration board usage recommendation
Figure 16. Efficiency vs. output current
100
Efficiency [%]
90
80
70
60
50
40
ST8R00
ST8R00W
Vin=5V , Vinh=5V, L=4.7µH,
Cin=10µF, Cout=10µF, Vout=8V
30
20
0
0.1
0.2
0.3
0.4
0.5 0.6
Iout [A]
0.7
0.8
0.9
1
Figure 17. Efficiency vs. output voltage
100
Efficiency [%]
95
90
85
80
75
70
65
ST8R00
ST8R00W
Vin=5V , Vinh=5V, L=4.7µH, Cin=10µF,
Cout=10µF, Iout=300mA
60
55
50
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
Vout [V]
Figure 18. ST8R00 efficiency vs. inductor
100
Efficiency [%]
90
80
70
60
50
Iout=300mA
Iout=0.5A
Iout=1A
Vin=5V , Vinh=5V, Vout=8V,
Cin=10µF, Cout=10µF
40
30
20
0
2
4
6
L [µH]
Doc ID 13913 Rev 2
8
10
15/19
BOM with most-used components
AN2627
Figure 19. ST8R00W efficiency vs. inductor
100
Efficiency [%]
90
80
70
60
50
Iout=300mA
Iout=0.5A
Iout=1A
Vin=5V , Vinh=5V, Vout=8V,
Cin=10µF, Cout=10µF
40
30
20
0
5
4
6
L [µH]
8
10
BOM with most-used components
Table 2.
16/19
2
Bill of materials
Name
Value
Material
Manufacturer
Part numbers
CIN
10 µF
Ceramic
Murata
GRM31CR61E106KA12B
COUT
10 µF
Ceramic
Murata
GRM31CR61E106KA12B
L
4.7 µH
Coiltronics
DR73-4R7
Doc ID 13913 Rev 2
AN2627
6
Footprint recommended data
Footprint recommended data
Figure 20. DFN8 4x4 recommended footprint
Doc ID 13913 Rev 2
17/19
Revision history
7
AN2627
Revision history
Table 3.
18/19
Document revision history
Date
Revision
Changes
13-May-2008
1
Initial release
03-Dec-2009
2
Modified Equation 9: on page 12.
Doc ID 13913 Rev 2
AN2627
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19/19
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