ST AN2681 Application note
AN2681
Application note
ST2S06A33 / ST2S06B
synchronous dual buck converter with reset
Introduction
The ST2S06 is a dual synchronous step-down DC-DC converter optimized for powering
low-voltage digital cores in ODD applications and, generally, used to replace the high
current linear solution when the power dissipation may cause a high heating of the
application environment. It provides up to 0.5 A over an input voltage range of 2.5 V to 5.5 V.
A high switching frequency (1.5 MHz) allows the use of tiny surface-mount components. A
resistor divider to set the output voltage value, an inductor and two capacitors are required
for every channel. In addition, a low output ripple is guaranteed by the current mode PWM
topology and by the use of low ESR surface-mount ceramic capacitors.
The device is thermal protected and current limited to prevent damage due to accidental
short-circuit.
The family is available in the QFN12L (4x4 mm) package.
Figure 1. ST2S06 - simplified schematic
* Only ST2S06A/D
** Only ST2S06B
September 2008 Rev 2 1/23
www.st.com
Contents AN2681
Contents
1 ST2S06 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Inhibit function (ST2S06B only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2 Reset function (ST2S06A and ST2S06D only) . . . . . . . . . . . . . . . . . . . . . 7
1.3 Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 Selecting components for applications . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.5 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 Demonstration board usage recommendation . . . . . . . . . . . . . . . . . . . 16
4.1 External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1.1 Capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5 Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6 Recommended footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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AN2681 List of figures
List of figures
Figure 1. ST2S06 - simplified schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 2. Inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 3. Output voltage ripple at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 4. Inductor current in PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 5. Output voltage ripple in PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 6. Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 7. ST2S06B - inhibit voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 8. ST2S06A\D - reset block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 9. ST2S06A\D - reset function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 10. Pull-up resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 11. ST2S06D - delay time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 12. Reset_in threshold vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 13. TDEL vs. temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 14. Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 15. Feedback voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 16. Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 17. Demonstration board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 18. Demonstration board - top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 19. Demonstration board - bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 20. Demonstration board schematic for ST2S06A\D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 21. Demonstration board schematic for ST2S06B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Figure 22. Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 23. Efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 24. Efficiency vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 25. QFN12L (4x4 mm) footprint recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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ST2S06 description AN2681
1 ST2S06 description
The ST2S06 is a dual adjustable current mode PWM synchronous step-down DC/DC
converter with an internal 0.5 A power switch. It is a complete 0.5 A dual switching regulator
with internal compensation that eliminates the need for additional components.
The device is available in three versions, the ST2S06A and ST2S06D with a reset function
and the ST2S06B with an inhibit function.
The ST2S06 family operates with typically 1.5 MHz fixed frequency.
To maintain good efficiency at both channels, the devices operate in power-save mode at
light load (Figure 2 and 3). When the load increases it automatically switches to PWM (pulse
width modulation) mode in order to reduce the output voltage ripple (Figure 4 and 5).
Figure 2. Inductor current at light load
Vin=5 V
V
=3.3 V
out1
V
=1.2 V
out2
=20 mA
I
out1
I
=100 mA
out2
4/23
AN2681 ST2S06 description
Figure 3. Output voltage ripple at light load
=5 V
V
in
V
=3.3 V
out1
V
=1.2 V
out2
R
R
LOAD1
LOAD2
=150 Ω
=15 Ω
Figure 4. Inductor current in PWM
Vin=5 V
V
=3.3 V
out1
V
=1.2 V
out2
I
=500 mA
out1
I
=500 mA
out2
5/23
ST2S06 description AN2681
Figure 5. Output voltage ripple in PWM
Vin=5 V
V
=3.3 V
out1
V
=1.2 V
out2
R
R
LOAD1
LOAD2
=10 Ω
=3.3 Ω
To clamp the error amplifier reference voltage, a Soft Start control block generating a voltage
ramp is implemented. When switching on the power supply, it allows controlling the inrush
current value.
Figure 6. Inrush current
Vin=5 V
V
=3.3 V
out1
V
=1.2 V
out2
R
R
LOAD1
LOAD2
=10 Ω
=3.3 Ω
Other protection circuits in the device are the thermal shutdown block which turns off the
regulator when the junction temperature exceeds 150 °C (typ.) and the cycle-by-cycle
current limiting that provides protection against shorted outputs.
Operation of the device requires few components: two inductors, three capacitors and two
resistor dividers. The inductors chosen must be capable of not saturating at the peak current
level. The value of the inductors should be selected keeping in mind that a large inductor
value increases the efficiency at low output current and reduces output voltage ripple, while
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AN2681 ST2S06 description
a smaller inductor can be chosen when it is important to reduce the package size and the
total application cost.
Finally, the ST2S06 family has been designed to work properly with X5R or X7R SMD
ceramic capacitors both at input and at output. These capacitors, thanks to their very low
series resistance (ESR), minimize the output voltage ripple. Other low ESR capacitors can
be used according to the need of the application without compromising the correct
functioning of the device.
Due to the high switching frequency and peak current, it is important to optimize the
application environment by reducing the length of the PCB traces and placing all external
components near the device.
1.1 Inhibit function (ST2S06B only)
The ST2S06B features an Inhibit function (pin 10). When the Inh voltage is higher than
1.3 V the device is On and if it is lower than 0.4 V the device is OFF. In shutdown mode
consumption is lower than 1 µA.
The Inh pin does not have an internal pull-up which means that you cannot leave the inhibit
floating.
If the inhibit function is not used, the Inh pin must be connected to V
Figure 7. ST2S06B - inhibit voltage vs. temperature
1.5
1.3
1.1
0.9
Vinh (V)
0.7
Vin=5V, Iout1,2=100mA
0.5
-50 -25 0 25 50 75 100 125
T [C°]
ON OFF
1.2 Reset function (ST2S06A and ST2S06D only)
Most ODD applications require a flag showing that the input voltage is in the correct range.
.
in
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