ST AN2728 Application note
AN2728
Application note
ST1S12
small synchronous buck converter
Introduction
The ST1S12 family of synchronous step-down DC-DC converters optimized for powering
low-voltage digital cores in HDD applications is generally used to replace the high-current
linear solution when the power dissipation may cause high heating of the application
environment. It provides up to 0.7 A over an input voltage range of 2.5 V to 5.5 V.
A high switching frequency (1.7 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 the adjustable version. Only an inductor and 2 capacitors are needed for the 1.2 V and
1.8 V fixed version. 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 damages due to accidental
short-circuit. This family of products is available in the TSOT23-5L package.
Figure 1. ST1S12 - simplified schematic
V
IN
VSUM
Σ
VC
g
m
+
-
SOFT START
REF1
Comp
FB/V
OSC
O
RST
V
VSET
UVLO
MOSFET
CONTROL
LOGIC
SHUT DOWN
EN
VDRV_P
VDRV_N
I_SEN
Driver
DMD
+
-
DMD
Driver
R
sense
SW
+
-
Gnd
April 2008 Rev 1 1/20
www.st.com
20
Contents AN2728
Contents
1 ST1S12 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Current limit and short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Selecting components for your applications . . . . . . . . . . . . . . . . . . . . . 7
2.1 Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4 Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.5 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Demonstration board usage recommendation . . . . . . . . . . . . . . . . . . . 13
4.1 External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2 Capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5 Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6 Recommended footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2/20
AN2728 List of figures
List of figures
Figure 1. ST1S12 - simplified schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 2. Inductor current at no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 3. Inductor current at medium load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 4. Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 5. Enable voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 6. Short-circuit protection simplified schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 7. Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 8. Drop vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 9. Feedback voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 10. Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 11. ST1S12 demonstration board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 12. Demonstration board layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 13. Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 14. Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 15. Efficiency vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 16. Efficiency vs. inductor at V
Figure 17. Efficiency vs. inductor at V
Figure 18. Efficiency vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 19. TSOT23-5L footprint dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
= 1.8 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
O
= 3.3 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
O
3/20
ST1S12 description AN2728
1 ST1S12 description
The ST1S12 is an adjustable current mode PWM synchronous step-down DC-DC converter
with an internal 0.7 A power switch. It is a complete 0.7 A switching regulator with internal
compensation which eliminates the need for additional components.
The device operates with typically 1.7 MHz fixed frequency, and in order to guarantee the
lowest switching ripple, operates in pulse width modulation (PWM) mode even at low-load
condition. (Figure 2 and Figure 3)
Figure 2. Inductor current at no load Figure 3. Inductor current at medium load
VEN=VIN=5 V, V
OUT
, CH4=I
CH2=V
=1.8 V, no load, CH1=SW,
OUT
L
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 4).
Figure 4. Inrush current
V
I
L
SW
OUT
(AC)
VEN=VIN=5 V, V
CH2=V
OUT
, CH4=I
OUT
L
V
EN
=1.8 V, R
OUT
=4.7 Ω, CH1=SW,
LOAD
Vout (AC)
I
L
SW
4/20
VEN=VIN=5 V, V
Ch1=SW, CH2=V
=1.8 V, R
OUT
, CH3=EN, CH4=I
OUT
LOAD
=15 Ω,
IN
SW
I
IN
AN2728 ST1S12 description
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.
The few components required for operation of the device are an inductor, two capacitors,
and a resistor divider. The inductor chosen must be capable of withstanding peak current
level without saturating. The value of the inductor should be selected keeping in mind that a
large inductor value increases the efficiency at low output current and reduces output
voltage ripple, while a smaller inductor can be chosen when it is important to reduce the
package size and the total application cost. The ST1S12 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 according to the need of the
application without compromising the right functioning of the device.
Finally, if the input voltage falls close to the output voltage, the ST1S12 can run at 100 %
duty cycle, in this mode the PMOS switch is continuously maintained ON. In this case the
output voltage value is the input voltage minus the voltage drop across the PMOS switch
and the resistance of the inductor.
The minimum input voltage to guarantee the right output voltage is:
where DCR
V
is DC resistance of the inductor and R
L
IN_MIN
= I
OUT_MAX
Due to the high switching frequency and peak current, it is important to optimize the
application environment such as reducing the length of the PCB traces and placing all
external components near the device.
1.1 Enable function
The ST1S12 features an enable function (pin 1). When the EN voltage is higher than 1.5 V
the device is ON and if it is lower than 0.5 V the device is OFF, Figure 5 shows the enable
voltage vs temperature. In shutdown mode consumption is lower than 1 µA.
The EN pin does not have an internal pull-up, which means that the enable pin cannot be
left floating.
If the enable function is not used, the EN pin must be connected to V
Figure 5. Enable voltage vs. temperature
1.8
1.8
1.6
1.6
1.4
1.4
1.2
1.2
[V]
[V]
1
1
EN
EN
0.8
0.8
V
V
0.6
0.6
0.4
0.4
0.2
0.2
0
0
-75 -50 -25 0 25 50 75 100 125 150 175
-75 -50 -25 0 25 50 75 100 125 150 175
V
V
IN
IN
= 5.5 V, I
= 5.5 V, I
OUT
OUT
= 10 mA
= 10 mA
x (R
T [°C]
T [°C]
DS(on)_P
+ DCRL) + V
DS(on)_P
is the resistance of the PMOS.
OUT
ON
ON
OFF
OFF
IN.
5/20
ST1S12 description AN2728
1.2 Current limit and short-circuit protection
In overcurrent protection mode, when the peak current reaches the current limit, the device
reduces t
reduced and, in most applications, this is enough to limit the current to I
In case of heavy short-circuit when the feedback voltage is lower than 0.1 V (typ.), the loop
switches to short mode automatically. In this condition the voltage V
compared with 0.4 V (typ.) to clamp the upper limit of the inductor current. In this condition
the maximum output limitation current is reduced to 300 mA instead of 1 A. At the same time
the DMD circuit clamps the lower boundary of the inductor current. One RS flip-flop is being
used to control the PMOS and NMOS switches. When the feedback voltage is higher than
0.1 V (typ.) voltage, the device returns to normal closed-loop switching operation (Figure 6).
Figure 6. Short-circuit protection simplified schematic
V
V
sum=Isen*Rsen
sum=Isen*Rsen
V
V
REF1
REF1
V
V
C
C
COM
COM
-
-
+
+
down to its minimum value. In these conditions, the duty cycle is strongly
ON
+
+
COM
COM
-
-
S
S
R
R
DMD
DMD
V
V
IN
IN
L1
Q
Q
-
-
+
+
L1
.
lim
sum=Isen
* R
R1
R1
R2
R2
sen
V
V
is
OUT
OUT
C2
C2
0.1V
0.1V
GND
V
V
FB
FB
GND
GND
GND
GND
GND
6/20
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