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PDF AN938 Data sheet ( Hoja de datos )
| Número de pieza | AN938 | |
| Descripción | Designing | |
| Fabricantes | ST Microelectronics | |
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AN938
® APPLICATION NOTE
DESIGNING WITH L4973, 3.5A HIGH EFFICIENCY
DC-DC CONVERTER
by N. Tricomi and G. Gattavari
INTRODUCTION
The L4973 family is a 3.5A monolithic dc-dc converter, step- down topology, operating in continuous
mode. It is realized in BCD60 II technology, and it’s available in two plastic packages, POWER-
DIP18(12+3+3) and SO20L (12+4+4).
Two versions are available, one fixing the output voltage, without any voltage divider, at 3.3V, and the
second at 5.1V.
Both the regulators can control higher output voltage values, by using an external voltage divider.
The operating input supply voltage is ranging from 8V to 55V, while the absolute value, with no load, is
60V.
New internal design solutions and superior technology performance allowed us to develop and produce
a device with improved efficiency in all the operating conditions and reduced external component
counts.
While internal limiting current and thermal shutdown are today considered standard protections func-
tions mandatory for a safe load supply, oscillator with voltage feedforward will improve line regulation
and overall control loop. Soft-start does not allow output overvoltages at turn-on, and synchronization
function can reduce EMI problems in multioutputs power supplies. Inhibit, introduced for power manage-
ment, in equipments having stand-by features, when active( high), reduces the device power consump-
tion, signal plus power stages, at few tens oDf aµtaAS. heet4U.com
DataShee
July 1998
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and is rapresented in figure 7:
AN938 APPLICATION NOTE
Figure 8. VO-IO Output characteristic.
et4U.com
Current protection
The L4973 has two current limiting, pulse by pulse
and hiccup mode. Increasing the output current till
the pulse by pulse limiting current threshold (Ith1 typ.
value of 4.5A) the controller reduces the on time,
maintaining the peak current at the value:
Ip
=
Ith1
+
(VCC
-
Vo
-
Ron
⋅
Ith1)
⋅
td
Lo
where td is the internal propagation delay of the cur-
rent protection loop (typical 300ns).
If the operating conditiones define a min on time
lower than td, the current increases to the following
value:
VO
D97IN686
Imax
=
(VCC
⋅
td ⋅ FSW ⋅ Vf ⋅ (1 − td ⋅
(Ro + Ron ⋅ td ⋅ FSW)
FSW))
4.5A 5.4A
IO
Where Ro is the load resistance, Vf is the diode forward voltage and Fsw is the switching frequency.
The output characteristic is rapresented in fig 8. At point A the output voltage drops, and the device is
going to pulse by pulse limiting current. Getting closer to the output short circuit, the current is shifting to
point B, a bit higher because of the ripple current reduction and because hiccup intervention, setted 20%
higher than pulse by pulse one.
Once the hiccup limiting current is operating, in output short circuit conditions the delivered average out-
put current is the value at point C.
Fig. 9 shows the internal current limiting cDircautaitSryh.eVeth41U.icsotmhe pulse by pulse while Vth2 is the hiccup
threshold.
Figure 9. Internal Current Limiting Schematic Diagram.
DataShee
OSC
SQ
R
VTh1
+
-
VTh2
+
-
OSC
VFB
PWM
+-
HICCUP
THERMAL
UNDERVOLTAGE
VREF
D97IN658
SOFT START
LATCH
SQ
R
+ 0.4
-
VCC
OUT
12V
CSS
The sense resistor is in series with a small mos realized as a partition of the main DMOS.
The Vth2 comparator (20% higher than Vth1) Sets the soft start latch, initializing the discharge of the soft
start capacitor at constant current (about 22µA).
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AN938 APPLICATION NOTE
TYPICAL APPLICATION.
Fig. 19 shows the typical application circuit, where the input supply voltage, Vcc, can range from 8 to
55V operating, and the output voltage adjustable from 3.3V to 40V.
The selected components, and in particular input and output capacitors, are able to sustain the device
voltage ratings, and the corresponding RMS currents.
Electrical Specification
Input Voltage Range
Output Voltage
Output Ripple
Output Current range
Max Output Ripple current
Current limit
Switching frequency
Target Efficiency
8V - 55V
5.1V ±3% (Line, Load and Thermal)
50mV
1mA - 3.5A
15% Iomax
4.5A
100kHz
85% @ 3.5A Vi = 50V
95% @ 0.5A Vi = 12V
Figure 19. Application circuit
VCC
R2
C1 C2 C7
(DIP18)
7,8
1
17 L4973
12
9
16 4,5,6 2,3
11 10 13,14,15
C8 L1
3x
C3 C4 C5 DatRa1 Sheet4U.com D1 C0
C6
VO
R3
C12
R4
C1=1000µF/63V
C2=220nF/63V
C3=470nF
C4=1µF/50V
C5=220pF
C6=22nF
C7=2.7nF
C8=220nF/63V
C0=100µF/40V(C9,C10,C11)
C12=Optional (220nF)
L1=150µH KOOLµ 77310 - 40 Turns - 0.9mm
R1=9.1K
R2=20K
D1=GI SB560
D97IN515B
L4973 V3.3
VO(V)
3.3
R3(KΩ)
0
R4(KΩ)
5.1 2.7 4.7
12 12 4.7
15 16 4.7
18 20 4.7
24 30 4.7
L4973 V5.1
VO(V)
5.1
R3(KΩ)
0
R4(KΩ)
12 6.2 4.7
15 9.1 4.7
18 12 4.7
24 18 4.7
Input Capacitor
The input capacitor has to be able to support the max input operating voltage of the device and the max
rms input current.
The input current is squared and the quality of these capacitors has to be very high to minimise its
power dissipation generated by the internal ESR, improving the system reliability. Moreover, input ca-
pacitors are also affecting the system efficiency.
The max Irms current flowing through the input capacitors is:
√Irms = Io ⋅
D
−
2
⋅ D2
η
+
D2
η2
where η is the expected system efficiency, D is the duty cycle and Io the output dc current.
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