ST AN2627 Application note
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
INH
IN
Thermal
Inhibit
Vref
LX
Ns
PGND
PWM control
PGND
Ps
GND
OUT
Po
FB
AM00001v1
December 2009 Doc ID 13913 Rev 2 1/19
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Contents AN2627
Contents
1 ST8R00 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 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. Simplified schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 2. ST8R00 inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 3. ST8R00W inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 4. ST8R00W inductor current at no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 5. ST8R00 cut-off block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 6. Current cut-off function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 7. Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 8. ST8R00 application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 9. Inhibit voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 10. Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 11. Voltage feedback vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 12. Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 13. The ST8R00 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 14. Demonstration board layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 15. Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 16. Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 17. Efficiency vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 18. ST8R00 efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 19. ST8R00W efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 20. DFN8 4x4 recommended footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Doc ID 13913 Rev 2 3/19
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
VIN=5 V, V
OUT
=8 V, I
=60 mA, Ch1=LX, Ch4=I
OUT
I
L
LX
L
4/19 Doc ID 13913 Rev 2
AN2627 ST8R00 description
Figure 3. ST8R00W inductor current at light load
I
L
LX
VIN=5 V, V
OUT
=8 V, I
=60 mA, Ch1=LX, Ch4=I
OUT
L
Figure 4. ST8R00W inductor current at no load
I
L
LX
VIN=5 V, V
=8 V, no load Ch1=LX, Ch4=I
OUT
L
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 AN2627
Figure 5. ST8R00 cut-off block
LX
LX
OUT
OUT
Ns
Ns
PGND
PGND
Figure 6. Current cut-off function
Ps
Ps
Po
Po
Iout
Vout
Inh
Vin
Figure 7 shows the in-rush current at start-up. Initially, the C
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’ R
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
capacitor is completely
OUT
DS(ON)
. As soon as
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