AN1518
Application note
Designing with the 2.5 A DC-DC converter L5973D
Introduction
The L5973D is a step-down monolithic power switching regulator capable of delivering up to
2.5 A at output voltages from 1.235 V to 35 V. The operating input voltage ranges from 4.4 V
to 36 V. It has been designed using BCDV technology and the power switching element is
implemented through a P-channel D-MOS transistor. It does not require a bootstrap
capacitor, and the duty cycle can range up to 100%. An internal oscillator fixes the switching
frequency at 250 kHz. This minimizes the LC output filter. A synchronization pin is available
for cases where a higher frequency (up to 500 kHz) is required. Pulse-by-pulse and
frequency foldback overcurrent protection offer effective short circuit protection. Other
features are voltage feed-forward, protection against feedback disconnection, inhibit and
thermal shutdown. The device is housed in an HSOP8 package with exposed pad that helps
to reduce the thermal resistance junction-to-ambient (R
Thj-a
) down to approximately
40 °C/W.
Figure 1.
Evaluation board
L5973D (HSOP8) board dimensions: 23x20 mm
Figure 2.
Package
Figure 3.
Pin connection
OUT
SYNC
INH
COMP
1
2
3
4
D98IN955
8
7
6
5
VCC
GND
VREF
FB
HSOP8 - Exposed Pad
January 2008
Rev 4
1/30
www.st.com
Contents
AN1518
Contents
1
Pin functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2
3
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
Power supply & voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Voltages monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Oscillator & synchronizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
PWM comparator and power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4
Additional features and protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1
4.2
4.3
Feedback disconnection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Zero load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5
6
7
8
9
Closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Error amplifier and compensation network . . . . . . . . . . . . . . . . . . . . . 11
LC filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
PWM comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1
9.2
9.3
9.4
Component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2/30
AN1518
Contents
9.5
Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10
Application ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1
10.2
10.3
10.4
10.5
10.6
Positive buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Dual output voltage with auxiliary winding . . . . . . . . . . . . . . . . . . . . . . . . 26
Synchronization example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Compensation network with MLCC (multiple layer ceramic capacitor) at the
output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
External SOFT_START network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
11
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3/30
List of figures
AN1518
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.
Evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Oscillator circuit block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Current limitation circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Driving circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Block diagram of the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Error amplifier equivalent circuit and compensation network . . . . . . . . . . . . . . . . . . . . . . . 12
Module plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Phase plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Layout example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Short-circuit current V
IN
= 25 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Short-circuit current V
IN
= 30 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Evaluation board application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
PCB layout (component side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
PCB layout (bottom side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
PCB layout (front side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Junction temperature vs. output current (V
CC
= 5 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Efficiency vs. output current (V
CC
= 5 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Junction temperature vs. output current (V
CC
= 12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Efficiency vs. output current (V
CC
=12 V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Positive buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Buck-boost regulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Dual output voltage with auxiliary winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Synchronization example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
MLCC compensation network example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Soft start network example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4/30
AN1518
Pin functions
1
1.1
Table 1.
N.
1
Pin functions
Pin description
Pin functions
Name
OUT
Regulator output
Master/slave synchronization. When open, a signal synchronous with the turn-OFF of the
internal power is present. When connected to an external signal at a frequency higher than
the internal one, the device is synchronized by the external signal.
Connecting the SYNC pins of two devices, the one with the higher frequency works as master
and the other one works as slave.
A logical signal (active high) disables the device. With an IHN higher than 2.2 V the device is
OFF and with an INH lower than 0.8 V, the device is ON.
If INH is not used, the pin must be grounded. When it is open, an internal pull-up disables the
device.
E/A output to be used for frequency compensation
Step-down feedback input. Connecting the output voltage directly to this pin results in an
output voltage of 1.235 V. An external resistor divider is required for higher output voltages
(the typical value for the resistor connected between this pin and ground is 4.7 k).
Reference voltage of 3.3 V. No filter capacitor is needed for stability
Ground
Unregulated DC input voltage
Description
2
SYNC
3
INH
4
5
6
7
8
COMP
FB
V
REF
GND
V
CC
2
Block diagram
Figure 4.
Block diagram
5/30
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