AN938
Application note
Designing with L4973, 3.5 A high efficiency
DC-DC converter
Introduction
The L4973 family is a 3.5 A monolithic DC-DC converter in step-down topology operating in
continuous mode. It uses BCD60 II technology and is available in two plastic packages,
power DIP18 (12+3+3) and SO20 (12+4+4).
Two versions of the device are available. The L4973V3.3 sets the output voltage without any
voltage divider at 3.3 V and the L4973V5.1 at 5.1 V.
Both the regulators can control higher output voltage values by using an external voltage
divider.
The operating input supply voltage ranges from 8 V to 55 V, while the absolute value with no
load is 60 V.
New internal design solutions and superior technological performance have allowed us to
develop and produce a device with improved efficiency in all operating conditions and with
fewer external components.
While internal current limiting and thermal shutdown are today considered standard
protection functions mandatory for a safe load supply, an oscillator with voltage feed-forward
improves line regulation and overall control loop. The soft-start function does not allow
output overvoltages at turn-on, and the synchronization function can reduce EMI problems
in multi-output power supplies. The inhibit function, introduced for power management in
equipment having standby features, when active (high), reduces device power consumption
by a few tens of µA.
A demonstration board for the L4973 is available through order code EVAL4973 (see
Figure 1).
Figure 1.
EVAL4973 demonstration board
AM03483v1
April 2009
Doc ID 5410 Rev 10
1/33
www.st.com
Contents
AN938
Contents
1
2
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Power supply, UVLO and voltage reference . . . . . . . . . . . . . . . . . . . . . . 6
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
Oscillator, sync and voltage feed-forward . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Feedback disconnection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Zero load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output overvoltage protection (OVP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Turn-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.9.1
2.9.2
Synchronization function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3
Typical application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
Electrical specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Output capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Error amplifier and compensation blocks . . . . . . . . . . . . . . . . . . . . . . . . . 22
LC filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
PWM gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.1
4.2
4.3
4.4
Mains transformer power supply with output adjustable from 0 V to 24 V 27
Higher input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Buck-boost converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Current generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2/33
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AN938
Contents
4.5
4.6
From positive input to negative output . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Negative boost converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Doc ID 5410 Rev 10
3/33
List of figures
AN938
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.
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.
L4973 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pin out - power DIP18 and SO20 packages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Device block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Turn-on sequence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Oscillator internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Device switching frequency vs. R
osc
and C
osc
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Voltage feed-forward function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Maximum duty cycle vs. R
osc
and C
osc
as parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Vo-Io output characteristics - hiccup protection limits output power . . . . . . . . . . . . . . . . . . . 9
Vo-Io output characteristics - effective pulse-by-pulse protection. . . . . . . . . . . . . . . . . . . . . 9
Schematic of internal current limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Output current and soft-start voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Maximum soft-start capacitance with f
SW
= 100 kHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Maximum soft-start capacitance with f
SW
= 200 kHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Soft-start internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Soft-start time vs. Vo and Css . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output rising voltage with Css 680 nF, 470 nF, 330 nF, 220 nF . . . . . . . . . . . . . . . . . . . . 13
Turn-on and turn-off (pin 2, 3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Power stage internal circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Sync and oscillator waveforms as slave and as master . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Re-start output voltage when inhibit goes low (Css = 56 nF) . . . . . . . . . . . . . . . . . . . . . . . 16
Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Input capacitance RMS current vs. duty cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Example of output regulated voltage lower than 3.3 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Output voltage vs. R5 using R3 as parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Ideal inductor value requested for 30% ripple current, as a function of max. input voltage
and output (f
SW
=150 kHz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Ideal inductor value requested for 30% ripple current, as a function of max. input voltage
and output (f
SW
=200 kHz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Ideal ripple voltage as a function of input and output voltage (f
SW
=150 kHz). . . . . . . . . . . 20
Output drop (%) vs minimum input voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Block diagram compensation loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
LC output filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Error amplifier equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Gain bode plot, open loop. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Phase bode plot, open loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Demonstration board (component side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Demonstration board (solder side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Typical adjustable 0-24 V power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Output voltage vs. R4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Design example for 70 V input supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Up/down converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Constant current generator up to 3.5 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Negative output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Negative boost converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4/33
Doc ID 5410 Rev 10
AN938
Description
1
Description
For a better understanding of the device and operation, a short description of the main
building blocks is given in
Figure 2
and
3
with packaging options and complete block
diagram.
Figure 2.
Pin out - power DIP18 and SO20 packages
OSC
OUT
OUT
GND
GND
GND
V
CC
V
CC
BOOT
1
2
3
4
5
6
7
8
9
D94IN162A
18
17
16
15
14
13
12
11
10
SYNC
SS
V5.1
GND
GND
GND
VFB
COMP
INH
OSC
OUT
OUT
GND
GND
GND
GND
V
CC
V
CC
BOOT
1
2
3
4
5
6
7
8
9
10
D94IN163A
20
19
18
17
16
15
14
13
12
11
SYNC
SS
V5.1
GND
GND
GND
GND
VFB
COMP
INH
Po werd ip (12+3+3)
SO20
(12+4+4)
AM03484v1
Figure 3.
Device block diagram
Doc ID 5410 Rev 10
5/33
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