ST1S14
Up to 3 A step-down switching regulator
Datasheet - production data
Applications
• Factory automation
• Printers
• DC-DC modules
+623H[SRVHGSDG
HSOP8
- exposed pad
• High current LED drivers
Features
Description
• 3 A DC output current
The ST1S14 is a step-down monolithic power
switching regulator able to deliver up to 3 A DC
current to the load depending on the application
conditions. The high current level is also achieved
thanks to a HSOP8 package with exposed frame,
that allows to reduce the Rth(JA) down to
approximately 40 °C/W. The output voltage can
be set from 1.22 V. The device uses an internal Nchannel DMOS transistor (with a typical RDS(on) of
200 mΩ) as the switching element to minimize the
size of the external components. The internal
oscillator fixes the switching frequency at 850
kHz. Power good open collector output validates
the regulated output voltage as soon as it reaches
the regulation. Pulse-by-pulse current limit offers
an effective constant current short-circuit
protection. Current foldback decreases overstress
in a persistent short-circuit condition.
• Operating input voltage from 5.5 V to 48 V
• 850 kHz internally fixed switching frequency
• Internal soft-start
• Power good open collector output
• Current mode architecture
• Embedded compensation network
• Zero load current operation
• Internal current limiting
• Inhibit for zero current consumption
• 2 mA maximum quiescent current over
temperature range
• 250 mΩ typ. RDS(on)
• Thermal shutdown
Figure 1. Application schematic
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December 2020
This is information on a product in full production.
DocID17977 Rev 3
1/46
www.st.com
46
Contents
ST1S14
Contents
1
2
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3
Enable inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3
ESD protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4
Function description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5
6
7
4.1
Power supply and voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.2
Voltage monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.3
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.4
Error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5
Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.6
Thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Additional features and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1
Maximum duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2
Minimum output voltage over VIN range . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1
GCO(s) control to output transfer function . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2
Error amplifier compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3
Voltage divider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.4
Total loop gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1
Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1.1
2/46
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
DocID17977 Rev 3
ST1S14
Contents
7.1.2
Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1.3
Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.2
Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.3
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.4
Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.5
7.4.1
300 mV < VFB < 1.22 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.4.2
VFB < 300 mV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.4.3
Start up phase in short circuit condition . . . . . . . . . . . . . . . . . . . . . . . . . 33
Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
8
Typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
9
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
10
Order code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
DocID17977 Rev 3
3/46
List of figure
ST1S14
List of figure
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.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
Figure 39.
Figure 40.
Figure 41.
Figure 42.
Figure 43.
Figure 44.
Figure 45.
4/46
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pin connection (top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Device block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Soft-start phases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Soft-start block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Bootstrap operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
VO_MIN over input voltage range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Block diagram of the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Transconductance embedded error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Leading network example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Module plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Phase plot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Layout example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Switching losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Minimum VFB for effective pulse-by-pulse protection over VIN . . . . . . . . . . . . . . . . . . . . . 30
IL diverging triggers hiccup protection (VIN = 48 V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Current and frequency foldback triggered when VFB IFOLD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Start up in short circuit condition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Over current protection triggers the frequency foldback . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Over current protection triggers the current and frequency foldback . . . . . . . . . . . . . . . . . 35
Demonstration board application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
PCB layout (component side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
PCB layout (bottom side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Line regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
RDSon vs. temperature (VIN = 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
VFB vs. temperature (VIN = 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
fSW vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Quiescent current vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Shutdown current vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Duty cycle max vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Efficiency vs. IOUT (VIN 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
TJ vs. IOUT (VIN 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Efficiency vs. IOUT (VIN 24 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
TJ vs. IOUT (VIN 24 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Efficiency vs. IOUT (VIN 32 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
TJ vs. IOUT (VIN 32 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
1 A to 3 A load transient (VIN 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Zoom - 1 A to 3 A load transient (VIN 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Zoom - 1 A to 3 A rising edge load transient (VIN 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
1 A to 3 A falling edge load transient (VIN 24 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
DocID17977 Rev 3
ST1S14
Figure 46.
Figure 47.
Figure 48.
Figure 49.
Figure 50.
Figure 51.
List of figure
Zoom - 1 A to 3 A rising edge load transient (VIN 24 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Zoom - 1 A to 3 A falling edge load transient (VIN 24 V). . . . . . . . . . . . . . . . . . . . . . . . . . . 41
1 A to 3 A load transient (VIN 32 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Zoom - 1 A to 3 A rising edge load transient (VIN 32 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Zoom - 1 A to 3 A falling edge load transient (VIN 32 V). . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Package dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
DocID17977 Rev 3
5/46
Pin settings
ST1S14
1
Pin settings
1.1
Pin connection
Figure 2. Pin connection (top view)
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1.2
Pin description
Table 1. Pin description
1.3
N
Pin
Description
1
BOOT
Bootstrap capacitor for N-channel gate driver. Connects 100 nF low ESR
capacitor from BOOT pin to SW
2
PG
Power good
3
EN1
Enable pin active low
4
FB
5
EN2
Enable pin active high
6
GND
Ground pin
7
VIN
Input supply pin
8
SW
Switching node
E.p.
Exposed pad must be connected to GND
Feedback voltage
Enable inputs
Table 2. Truth table
6/46
EN1
EN2
Device status
H
L
INH
H
H
INH
L
L
INH
L
H
ON
DocID17977 Rev 3
ST1S14
Electrical data
2
Electrical data
2.1
Maximum ratings
Table 3. Absolute maximum ratings
Symbol
Value
Unit
Power supply input voltage
-0.3 to 52
V
VEN1
Enable 1 voltage
-0.3 to 7
V
VEN2
Enable 2 voltage
-0.3 to (VIN+0.3)
V
PG
Power good
-0.3 to (VIN+0.3)
V
BOOT
Bootstrap pin
-0.3 to 55
V
-1 to (VIN+0.3)
V
-0.3 to 3
V
Operating junction temperature range
-40 to 150
°C
TSTG
Storage temperature range
-65 to 150
°C
TLEAD
Lead temperature (soldering 10 sec.)
260
°C
Value
Unit
40
°C/W
Value
Unit
HBM
4
kV
MM
500
V
VIN
SW
Switching node
VFB
Feedback voltage
TJ
2.2
Parameter
Thermal data
Table 4. Thermal data
Symbol
Rth JA
2.3
Parameter
Thermal resistance junction-ambient
ESD protection
Table 5. ESD protection
Symbol
ESD
Test condition
DocID17977 Rev 3
7/46
Electrical characteristics
3
ST1S14
Electrical characteristics
All the population tested at TJ = 25 °C, VCC =12 V, VEN1 = 0 V, VEN2 = VCC unless otherwise
specified.
The specification is guaranteed from (-40 to +125 °C) TJ temperature range by design,
characterization, and statistical correlation.
Table 6. Electrical characteristics
Symbol
VIN
RDS(on)
ISW
Parameter
Test condition
Min
Operating input
voltage range
MOSFET on
resistance
Max
Unit
48
V
0.2
0.4
Ω
4.5
5.2
A
5.5
ISW=1 A
Maximum limiting
current
3.7
tHICCUP Hiccup time
fSW
Typ
16
Switching frequency
600
850
ms
1000
kHz
Duty cycle
(1)
90
%
Minimum conduction
TON MIN time of the power
element
(1)
90
ns
Minimum conduction
TOFF MIN time of the external
diode
(1)
75
90
120
ns
ILOAD=0 A
1.202
1.22
1.239
V
ILOAD=10 mA to 3 A
1.196
1.22
1.245
V
DC characteristics
VFB
Voltage feedback
IFB
FB biasing current
Iq
Quiescent current
Iqst-by
Standby quiescent
current
50
VFB=2 V
1.3
2
mA
VFB=2 V, VIN=48 V
1.7
2.4
mA
Device OFF (see Table 2)
16
34
µA
VFB rising edge
0.92*
VOUT
V
VFB falling edge
0.8*
VOUT
V
Power good threshold
PG
PG output voltage
I
=6 mA
(open collector active) SINK
Inhibit
8/46
nA
DocID17977 Rev 3
0.4
V
ST1S14
Electrical characteristics
Table 6. Electrical characteristics (continued)
Symbol
VEN1
IEN1
VEN2
IEN2
Parameter
Enable 1 levels
Enable 1 biasing
current
Enable 2 levels
Enable 2 biasing
current
Test condition
Min
Typ
Device ON
VIN=5.5 V to 48 V
Device OFF
VIN=5.5 V to 48 V
1.5
VEN1=5 V
0.7
Device ON
VIN=5.5 V to 48 V
1.5
Max
Unit
0.5
V
V
1.6
3.5
µA
V
Device OFF
VIN=5.5 V to 48 V
0.5
V
VEN1=0 V; VEN2=0 V
-1
-2.4
-4.5
µA
VEN1=0 V; VEN2=12 V
2.7
5.8
10
µA
VEN1=0 V; VCC=VEN2=48 V
3.0
6.0
10
µA
140
150
160
°C
Thermal shutdown
TSHDWN
Thermal shutdown
temperature
(1)
THYS
Thermal shutdown
hysteresis
(1)
15
°C
1. Parameter guaranteed by design
DocID17977 Rev 3
9/46
Function description
4
ST1S14
Function description
The ST1S14 is based on a “peak current mode”, constant frequency control. As a
consequence the intersection between the error amplifier output and the sensed inductor
current generates the control signal to drive the power switch.
The main internal blocks shown in the block diagram in Figure 3 are:
•
A fully integrated sawtooth oscillator with a typical frequency of 850 kHz
•
A transconductance error amplifier
•
A high side current sense amplifier to track the inductor current
•
A pulse width modulator (PWM) comparator and the circuitry necessary to drive the
internal power element
•
Soft-start circuitry to decrease the inrush current at power-up
•
Current limitation circuit based on the pulse-by-pulse current protection with frequency
divider based on FB voltage and the hiccup protection
•
Bootstrap circuitry to drive the embedded N-MOS switch
•
A multi input inhibit block for standby operation
•
A circuit to implement the thermal protection function
Figure 3. Device block diagram
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DocID17977 Rev 3
ST1S14
4.1
Function description
Power supply and voltage reference
The internal regulator circuit consists of a start-up circuit, an internal voltage pre-regulator,
the bandgap voltage reference, and the bias block that provides current to all the blocks.
The starter supplies the start-up current to the entire device when the input voltage goes
high and the device is enabled (inhibit pin connected to ground). The pre-regulator block
supplies the bandgap cell with a pre-regulated voltage that has a very low supply voltage
noise sensitivity.
4.2
Voltage monitor
An internal block continuously senses the Vcc, Vref, and Vbg. If the monitored voltages are
good, the regulator begins operating. There is also a hysteresis on the VCC (UVLO).
Figure 4. Internal circuit
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4.3
Soft-start
The startup phase minimizes the inrush current and decreases the stress of the power
components at the power up. The startup takes place when VIN crosses the selected UVLO
threshold. A internal counter (2816 clks) sets the soft start time (see Figure 5).
The reference of the error amplifier is ramped smootly in 704 steps (one step every 4 clks).
A low pass filter smooths each step to minimize output discontinuity. Considering the typical
850 kHz switching frequency, the phase two duration is 3.3 msec
The device has full load current capability during the soft start time in order to charge the
output capacitor (see Figure 5).
DocID17977 Rev 3
11/46
Function description
ST1S14
Figure 5. Soft-start phases
During normal operation a new soft start cycle takes place in case of:
•
HICCUP mode current protection
•
thermal shutdown event
•
UVLO event
•
the device is driven in INH mode
Figure 6. Soft-start block diagram
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DocID17977 Rev 3
ST1S14
4.4
Function description
Error amplifier
The voltage error amplifier is the core of the loop regulation. It is a transconductance
operational amplifier whose non inverting input is connected to the internal voltage
reference (1.222 V), while the inverting input (FB) is connected to the external divider or
directly to the output voltage.
The error amplifier is internally compensated to minimize the size of the final application.
Table 7. Uncompensated error amplifier characteristics
Description
Values
Transconductance
218 µS
Low frequency gain
93 dB
CP
24 pF
CC
211 pF
RC
200 kΩ
The error amplifier output is compared with the inductor current sense information to
perform PWM control.
4.5
Inhibit function
The inhibit feature is used to set the device in standby mode according to Table 2. When the
device is disabled, the power consumption is reduced to less than 40 µA. The EN2 pin is
also VIN compatible.
4.6
Thermal shutdown
The shutdown block generates a signal that turns off the power stage if the temperature of
the chip goes higher than a fixed internal threshold (150±10 °C). The sensing element of the
chip is very close to the PDMOS area, ensuring fast and accurate temperature detection. A
hysteresis of approximately 15 °C keeps the device from turning on and off continuously.
DocID17977 Rev 3
13/46
Additional features and limitations
ST1S14
5
Additional features and limitations
5.1
Maximum duty cycle
The bootstrap circuitry charges, cycle-by-cycle, the external bootstrap capacitor to generate
a voltage higher than VIN necessary to drive the internal N-channel power element.
An internal linear regulator charges the CBOOT during the conduction time of the external
freewheeling diode during the switching activity. The internal logic implements a minimum
OFF time of the high side switch (90 nsec typ.) to prevent the bootstrap discharge at high
duty cycle. As a consequence, the ST1S14 can operate at a maximum duty cycle of around
90 % typ.
The ST1S14 embeds the diode VD1 required for the bootstrap operation.
Figure 7. Bootstrap operation
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14/46
DocID17977 Rev 3
ST1S14
5.2
Additional features and limitations
Minimum output voltage over VIN range
The minimum regulated output voltage at a given input voltage is limited by the minimum
conduction time of the power element, that is 90 nsec typ. for the ST1S14:
Equation 1
T ON_MIN
90ns
V O_MIN ( V IN ) = V IN ⋅ D MIN = V IN ⋅ ---------------------- = V IN ⋅ -----------------T SW
1.18µs
which is plotted in Figure 14. The reference of the embedded error amplifier (1.22 V) sets
the minimum VO_SET at low VIN.
Figure 8. VO_MIN over input voltage range
$09
Figure 8 shows the minimum output voltage over input voltage range to have constant
switching activity and a predictable output voltage ripple.
The regulator can, however, regulate the minimum input voltage over the entire input
voltage range but, given the 90 ns minimum conduction time of the power element, it skips
some pulses to keep the output voltage in regulation when Equation 1 is not satisfied.
This operation is not recommended at the nominal input voltage of the application mainly
because it affects the output voltage ripple, but it is generally accepted during a line
transient event.
DocID17977 Rev 3
15/46
Closing the loop
6
ST1S14
Closing the loop
Figure 9. Block diagram of the loop
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16/46
DocID17977 Rev 3
ST1S14
6.1
Closing the loop
GCO(s) control to output transfer function
The accurate control to output transfer function for a buck peak current mode converter can
be written as:
Equation 2
s
1 + ----
ω z
R0
1
G CO ( s ) = ------- ⋅ ---------------------------------------------------------------------------------------- ⋅ ---------------------- ⋅ F H ( s )
Ri
R 0 ⋅ T SW
s
1 + ----------------------- ⋅ [ m C ⋅ ( 1 – D ) – 0.5 ] 1 + -----ω p
L
where R0 represents the load resistance, Ri the equivalent sensing resistor of the current
sense circuitry, ωp the single pole introduced by the LC filter, and ωz the zero given by the
ESR of the output capacitor.
FH(s) accounts for the sampling effect performed by the PWM comparator on the output of
the error amplifier that introduces a double pole at one half of the switching frequency.
Equation 3
1
ω Z = ------------------------------ESR ⋅ C OUT
Equation 4
m C ⋅ ( 1 – D ) – 0.5
1
ω n = -------------------------------------- + --------------------------------------------L ⋅ C OUT ⋅ fSW
R LOAD ⋅ C OUT
where:
Equation 5
Se
m C = 1 + -----Sn
S = V ⋅ f
pp SW
e
V
IN – V OUT
S = ----------------------------- ⋅ Ri
n
L
Sn represents the ON time slope of the sensed inductor current, and Se the ON time slope
of the external ramp (VPP peak to peak amplitude) that implements the slope compensation
to avoid sub-harmonic oscillations at duty cycle over 50 %.
The sampling effect contribution FH(s) is:
Equation 6
1
F H ( s ) = -----------------------------------------2
s
s
1 + ------------------- + ------2
ω n ⋅ QP ω
n
where:
DocID17977 Rev 3
17/46
Closing the loop
ST1S14
Equation 7
1
Q P = ---------------------------------------------------------π ⋅ [ m C ⋅ ( 1 – D ) – 0.5 ]
6.2
Error amplifier compensation network
The ST1S14 embeds the error amplifier (see Figure 10) and a pre-defined compensation
network which is effective in stabilizing the system in most of the application conditions.
Figure 10. Transconductance embedded error amplifier
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RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect
system stability but it is useful to reduce the noise at the output of the error amplifier.
The transfer function of the error amplifier and its compensation network is:
Equation 8
A V0 ⋅ ( 1 + s ⋅ R c ⋅ C c )
A 0 ( s ) = ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2
s ⋅ R 0 ⋅ ( C 0 + C p ) ⋅ R c ⋅ C c + s ⋅ ( R0 ⋅ C c + R 0 ⋅ ( C 0 + C p ) + R c ⋅ C c ) + 1
where Avo = Gm · Ro.
The poles of this transfer function are (if Cc >> C0+CP):
Equation 9
1
f P LF = ---------------------------------2 ⋅ π ⋅ R0 ⋅ Cc
18/46
DocID17977 Rev 3
ST1S14
Closing the loop
Equation 10
1
fP HF = ---------------------------------------------------2 ⋅ π ⋅ Rc ⋅ ( C0 + Cp )
whereas the zero is defined as:
Equation 11
1
F Z = --------------------------------2 ⋅ π ⋅ Rc ⋅ Cc
The embedded compensation network is RC=200 K, CP=24 pF, CC=211 pF and CO can be
considered negligible, so the singularities are:
Equation 12
f Z = 3, 77 kHz
6.3
f P LF = 3, 01 Hz
f P HF = 33, 16 kHz
Voltage divider
The contribution of a simple voltage divider is:
Equation 13
R2
G DIV ( s ) = -------------------R1 + R2
Figure 11. Leading network example
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A small signal capacitor in parallel to the upper resistor (see Figure 11.) of the voltage
divider implements a leading network (fzero < fpole), sometimes necessary to improve the
system phase margin:
Equation 14
DocID17977 Rev 3
19/46
Closing the loop
ST1S14
R2
( 1 + s ⋅ R 1 ⋅ C R1 )
G DIV ( s ) = -------------------- ⋅ ----------------------------------------------------------R1 + R2
R1 ⋅ R2
-------------------- ⋅ C R1
1 + s ⋅ R
1 + R2
where:
1
f Z = -------------------------------------2 ⋅ π ⋅ R 1 ⋅ C R1
1
fP = --------------------------------------------------R 1 ⋅ R2
2 ⋅ π ⋅ -------------------- ⋅ C R1
R1 + R2
fZ < f P
6.4
Total loop gain
In summary, the open loop gain can be expressed as:
Equation 15
G ( s ) = G DIV ( s ) ⋅ G CO ( s ) ⋅ A 0 ( s )
Example: VIN = 12 V, VOUT = 3.3 V, ROUT = 2 Ω.
The resistor divider is R1=5.6 K, R2=3.3 K.
CR1=150 nF implements a leading network (fZ=190 kHz, fP=510 kHz).
Selecting L = 8.2 µH, COUT = 100 µF, and ESR = 75 mΩ, the gain and phase bode
diagrams are plotted respectively in Figure 12 and 13 over input voltage range (VIN=6
V to 48 V, IOUT=3 A).
Figure 12. Module plot
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DocID17977 Rev 3
ST1S14
Closing the loop
Figure 13. Phase plot
The cut-off frequency and the phase margin are:
Equation 16
V IN = 6V
f C = 46 kHz
pm = 49°
V IN = 12V
f C = 71 kHz
pm = 62°
V IN = 48V
f C = 97 kHz
pm = 78°
DocID17977 Rev 3
21/46
Application information
ST1S14
7
Application information
7.1
Component selection
7.1.1
Input capacitor
The input capacitor must be able to support the maximum input operating voltage and the
maximum RMS input current.
Since step-down converters draw current from the input in pulses, the input current is
squared and the height of each pulse is equal to the output current. The input capacitor has
to absorb all this switching current, whose RMS value can be up to the load current divided
by two (worst case, with duty cycle of 50 %). For this reason, the quality of these capacitors
must be very high to minimize the power dissipation generated by the internal ESR, thereby
improving system reliability and efficiency. The critical parameter is usually the RMS current
rating, which must be higher than the RMS current flowing through the capacitor. The
maximum RMS input current (flowing through the input capacitor) is:
Equation 17
2
2
2⋅D
D
I RMS = I O ⋅ D – --------------- + ------2η
η
where η is the expected system efficiency, D is the duty cycle, and IO is the output DC
current. Considering η = 1 this function reaches its maximum value at D = 0.5 and the
equivalent RMS current is equal to IO divided by 2. The maximum and minimum duty cycles
are:
Equation 18
V OUT + VF
D MAX = ------------------------------------V INMIN – V SW
and
Equation 19
VOUT + V F
D MIN = -------------------------------------V INMAX – V SW
Where VF is the freewheeling diode forward voltage and VSW the voltage drop across the
internal PDMOS. Considering the range DMIN to DMAX, it is possible to determine the
maximum IRMS going through the input capacitor. Capacitors that may be considered are:
Electrolytic capacitors:
These are widely used due to their low cost and their availability in a wide range of
RMS current ratings.
The only drawback is that, considering ripple current rating requirements, they are
physically larger than other capacitors.
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DocID17977 Rev 3
ST1S14
Application information
Ceramic capacitors:
If available for the required value and voltage rating, these capacitors usually have a
higher RMS current rating for a given physical dimension (due to very low ESR).
The drawback is the considerably high cost.
Tantalum capacitors:
Small tantalum capacitors with very low ESR are becoming more available. However,
they can occasionally burn if subjected to very high current during charge.
Therefore, it is suggested to avoid this type of capacitor for the input filter of the device
as they could be stressed by a high surge current when connected to the power supply.
Table 8. List of ceramic capacitors for the ST1S14
Manufacturer
Series
Capacitor value (µ)
Rated voltage (V)
TAIYO YUDEN
UMK325BJ106MM-T
10
50
MURATA
GRM42-2 X7R 475K 50
4.7
50
If the selected capacitor is ceramic (so neglecting the ESR contribution), the input voltage
ripple can be calculated as:
Equation 20
IO
D
D
V IN PP = ----------------------- ⋅ 1 – ---- ⋅ D + ---- ⋅ ( 1 – D )
C IN ⋅ fSW
η
η
7.1.2
Output capacitor
The output capacitor is very important to meet the output voltage ripple requirement.
Using a small inductor value is useful to reduce the size of the choke but it increases the
current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is required.
Nevertheless, the ESR of the output capacitor introduces a zero in the open loop gain,
which helps to increase the phase margin of the system. If the zero goes to a very high
frequency, its effect is negligible.
Ceramic capacitors
Ceramic capacitors and very low ESR capacitors that introduce a zero outside the
designed bandwidth (fZ=1/(2*pi*ESR*COUT, see Section 6: Closing the loop) in general
should be avoided. A leading network across the upper resistor of the voltage divider is
useful to increase the phase margin and compensate the system (see Section 6.3:
Voltage divider). The effectiveness of the leading network increases at high output
voltage because the singularities become more split.
High ESR capacitors
The “high ESR capacitor” definition stands for a capacitor having an ESR value able to
introduce a zero into the designed system bandwidth, which can be, as a general rule,
up to fSW/5 at maximum. Tantalum or electrolytic capacitors belong to this group.
Equation 21
f SW
1
f Z = -------------------------------------------------- < BW < --------2 ⋅ π ⋅ ESR ⋅ COUT
5
DocID17977 Rev 3
23/46
Application information
ST1S14
A list of some tantalum capacitor manufacturers is provided in Table 9.
Table 9. Output capacitor selection
Manufacturer
Series
Rated voltage (V)
Nippon Chemicon
KZE
6.3 to 50
TAE
4 to 16
THB/C/E
4 to 16
TPS
4 to 35
Sanyo
POSCAP(2)
AVX
Cap value (µF)(1)
ESR (mΩ)(1)
1
f Z = -------------------------------------------------- < BW
2 ⋅ π ⋅ ESR ⋅ COUT
1. see Section 6: Closing the loop for the selection of the output capacitor
2. POSCAP capacitors have some characteristics which are very similar to tantalum.
7.1.3
Inductor
The inductor value is very important as it fixes the ripple current flowing through the output
capacitor. The ripple current is usually fixed at 20 - 40 % of Iomax, which is 0.6 - 1.2 A with
IOmax = 3 A. The approximate inductor value is obtained using the following formula:
Equation 22
( VIN – V OUT )
L = ---------------------------------- ⋅ T ON
∆I
where TON is the ON time of the internal switch, given by D · T. For example, with
VOUT = 3.3 V, VIN = 24 V, and ∆IO = 0.8 A, the inductor value is about 4.7 µH. The peak
current through the inductor is given by:
Equation 23
∆I
I PK = I O + ----2
and it can be observed that if the inductor value decreases, the peak current (which must be
lower than the current limit of the device) increases. So, when the peak current is fixed, a
higher inductor value allows a higher value for the output current. In Table 10, some inductor
manufacturers are listed.
Table 10. Inductor selection
Manufacturer
Wurth Elektronik
Coilcraft
7.2
Series
Inductor value (µH)
Saturation current (A)
WE-HCI 7040
1 to 4.7
20 to 7
WE-HCI 7050
4.9 to 10
20 to 4.0
XPL 7030
2.2 to 10
29 to 7.2
Layout considerations
The layout of switching DC-DC converters is very important to minimize noise and
interference. Power-generating portions of the layout are the main cause of noise and so
24/46
DocID17977 Rev 3
ST1S14
Application information
high switching current loop areas should be kept as small as possible and lead lengths as
short as possible.
High impedance paths (in particular the feedback connections) are susceptible to
interference, so they should be as far as possible from the high current paths. A layout
example is provided in Figure 14 below.
The input and output loops are minimized to avoid radiation and high frequency resonance
problems. The feedback pin connections to the external divider are very close to the device
in order to avoid pick-up noise. Another important issue is the ground plane of the board. As
the package has an exposed pad, it is very important to connect it to an extended ground
plane in order to reduce the thermal resistance junction-to-ambient.
To increase the design noise immunity, different signal and power ground should be
implemented in the layout (see Section 7.5: Application circuit). The signal ground serves
the small signal components, the device ground pin, the exposed pad, and a small filtering
capacitor connected to the VCC pin. The power ground serves the external diode and the
input filter. The different grounds are connected underneath the output capacitor. Neglecting
the current ripple contribution, the current flowing through this component is constant during
the switching activity and so this is the cleanest ground point of the buck application circuit.
Figure 14. Layout example
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DocID17977 Rev 3
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Application information
7.3
ST1S14
Thermal considerations
The dissipated power of the device is tied to three different sources:
•
Conduction losses due to the not insignificant RDSON, which are equal to:
Equation 24
2
P ON = R DSON ⋅ ( IOUT ) ⋅ D
where D is the duty cycle of the application. Note that the duty cycle is theoretically given by
the ratio between VOUT and VIN, but in practice it is substantially higher than this value to
compensate for the losses in the overall application. For this reason, the conduction losses
related to the RDSON increase compared to an ideal case.
•
Switching losses due to turning on and off. These are derived using the following
equation:
Equation 25
( T RISE + T FALL )
P SW = V IN ⋅ I OUT ⋅ ----------------------------------------- ⋅ F SW = VIN ⋅ I OUT ⋅ T SW_EQ ⋅ F SW
2
where TRISE and TFALL represent the switching times of the power element that cause the
switching losses when driving an inductive load (see Figure 15). TSW is the equivalent
switching time.
Figure 15. Switching losses
$09
•
Quiescent current losses.
Equation 26
P Q = V IN ⋅ I Q
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DocID17977 Rev 3
ST1S14
Application information
Example:
–
VIN = 24 V
–
VOUT = 5 V
–
IOUT = 3 A
RDS(on) has a typical value of 0.2 Ω @ 25 °C and increases to a maximum value of 0.4 Ω @
125 °C. We can consider a value of 0.3 Ω.
TSW_EQ is approximately 12 ns.
IQ has a typical value of 2 mA @ VIN = 24 V.
The overall losses are:
Equation 27
2
P TOT = R DSON ⋅ ( I OUT ) ⋅ D + V IN ⋅ I OUT ⋅ T SW ⋅ F SW + V IN ⋅ I Q = ""
2
= 0.3 ⋅ ( 3 ) ⋅ 0.137 + 24 ⋅ 3 ⋅ 12 ⋅ 10
–9
⋅ 850 ⋅ 10
–3
+ 24 ⋅ 2 ⋅ 10
–3
≅ 1.15W
The junction temperature of the device is:
Equation 28
T J = T A + Rth J – A ⋅ P TOT
where TA is the ambient temperature and RthJ-A is the thermal resistance junction-toambient. Considering that the device is mounted on board with a good ground plane, that it
has a thermal resistance junction-to-ambient (RthJ-A) of about 40 °C/W, and an ambient
temperature of about 40 °C:
T J = 40 + 1.15 ⋅ 40 ≅ 86°C
7.4
Short-circuit protection
In overcurrent protection mode, when the peak current reaches the current limit, the device
disables the power element and it is able to reduce the conduction time down to the
minimum value (approximately 90 nsec typical) to keep the inductor current limited. This is
the pulse by pulse current limitation to implement constant current protection feature.
For the ST1S14, the operation of the pulse by pulse current limitation out of the soft start
time depends on the FB voltage:
•
300 mV
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