AN557
APPLICATION NOTE
EASY APPLICATION DESIGN WITH THE L4970A,
MONOLITHIC DC-DC CONVERTERS FAMILY
The L497XA series of high current switching regulator ICs exploit Multipower-BCD technology to
achieve very high output currents with low power dissipation – up to 10A in the Multiwatt power package
and 3.5A in a DIP package .
THE TECHNOLOGY
The technology architecture is based on the vertical DMOS silicon gate process that allows a channel
length of 1.5 micron ; using a junction isolation technique it has been possible to mix on the same chip
Bipolar and CMOS transistors along with the DMOS power components (Fig. 2). Figure 1 shows how this
process brings a rapid increase in power IC complexity compared to conventional bipolar technology. In
the 70’s class B circuits and DC circuits allowed output power in the range of 70W. By 1980, with the in-
troduction of switching techniques in power ICs, output powers up to 200W were reached ; with BCD tech-
nology the output power increased up to 400W.
November 2003
1/52
AN557 APPLICATION NOTE
Figure 1. BCD process and increase in power ICs complexity.
Figure 2. Cross Section of the BCD Mixed Technology.
THE STEP-DOWN CONFIGURATION
Fig. 3 shows the simplified block diagram of the circuit realizing the step-down configuration. This circuit
operates as follows : Q1 acts as a switch at the frequency f and the ON and OFF times are suitably con-
trolled by the pulse width modulator circuit. When Q1 is saturated, energy is absorbed from the input which
is transferred to the output through L. The emitter voltage of Q1, V
E
, is Vi-Vsat when Q is ON and -V
F
(with
VF the forward voltage across the D diode as indicated) when Q1 is OFF. During this second phase the
current circulates again through L and D. Consequently a rectangular shaped voltage appears on the emit-
ter of Q1 and this is then filtered by the L-C-D network and converted into a continuous mean value across
the capacitor C and therefore across the load. The current through L consists of a continuous component,
I
LOAD
, and a triangular-shaped component super-imposed on it,
∆I
L
, due to the voltage across L.
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AN557 APPLICATION NOTE
Figure 3. The Basic Step-down Switching Regulator Configuration
Fig. 4 shows the behaviour of the most significant waveforms, in different points of the circuit, which help
to understand better the operation of the power section of the switching regulator. For the sake of simplic-
ity, the series resistance of the coil has been neglected. Fig. 2a shows the behaviour of the emitter voltage
(which is practically the voltage across the recirculation diode), where the power saturation and the for-
ward VF drop across the diode era taken into account.
The ON and OFF times are established by the following expression :
T
O N
-
V
o
=
(
V
i
–
V
sa t
) ⋅
-------------------------------
T
+
T
ON
O FF
Fig. 4b shows the current across the switching transistor. The current shape is trapezoidal and the oper-
ation is in continuous mode. At this stage, the phenomena due to the catch diode, that we consider as
dynamically ideal, are neglected. Fig. 4c shows the current circulating in the recirculation diode. The sum
of the currents circulating in the power and in the diode is the current circulating in the coil as shown in
Fig. 4e. In balanced conditions the
∆
IL+
current increase occuring during T
ON
has to be equal to the
∆I
L–
decrease occurring during T
OFF
. The mean value of I
L
corresponds to the charge current. The current rip-
ple is given by the following formula :
(
V
i
–
V
sat
)
–
V
o
V
o
+
V
F
+
-
-
-
∆
I
L
=
∆
I
L
= ---------------------------------------
T
O N
= -------------------
T
O FF
L
L
It is a good rule to respect to I
oMIN
≥
I
L
/2 relationship, that implies good operation in continuous mode.
When this is not done, the regulator starts operating in discontinuous mode. This operation is still safe but
variations of the switching frequency may occur and the output regulation decreases.
Fig. 4d shows the behaviour of the voltage across coil L. In balanced conditions, the mean value of the
voltage across the coil is zero. Fig. 4f shows the current flowing through the capacitor, which is the differ-
ence between I
L
and I
LOAD
.
In balanced conditions, the mean current is equal to zero, and
∆I
C
=
∆I
L
. The current I
C
through the ca-
pacitor gives rise to the voltage ripple.
This ripple consists of two components : a capacitive component,
∆V
C
, and a resistive component,
∆V
ESR
,
due to the ESR equivalent series resistance of the capacitor. Fig. 4g shows the capacitive component
∆V
C
of the voltage ripple, which is the integral of a triangular-shaped current as a function of time. Moreover,
it should be observed that v
C
(t) is in quadrature with i
C
(t) and therefore with the voltage V
ESR
. The quantity
of charge
∆Q
+
supplied to the capacitor is given by the area enclosed by the ABC triangle in Fig. 4f :
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AN557 APPLICATION NOTE
Figure 4. Principal Circuit Waveforms of the figure 1 Circuit.
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AN557 APPLICATION NOTE
1 T
∆
I
L
-
- -
∆
Q
= --
⋅
--
⋅
-------
2 2 2
Which therefore gives:
∆
I
L
Q
∆
V
C
= --- = --------
-
8 fc
C
Fig. 4h shows the voltage ripple V
ESR
due to the resistive component of the capacitor. This component is
V
ESR
(t) = i
C
(t) × ESR. Fig. 4i shows the overall ripple Vo, which is the sum of the two previous compo-
nents. As the frequency increases (> 20kHz), which is required to reduce both the cost and the sizes of L
and C, the VESR component becomes dominant. Often it is necessary to use capacitors with greater ca-
pacitance (or more capacitors connected in parallel to limit the value of ESR within the required level.
We will now examine the stepdown configuration in more detail, referring to fig. 1 and taking the be-haviour
shown in Fig. 4 into account.
Starting from the initial conditions, where Q = ON, v
C
= V
o
and i
L
= i
D
= 0, using Kirckoff second principle
we may write the following expression:
V
i
= v
L
+ v
C
(V
sat
is neglected against V
i
).
d
IL
d
IL
-
V
i
=
L
------- +
v
C
=
L
⋅
------- +
V
o
(1)
-
dt
dt
which gives :
d
IL
(
V
i
–
V
o
)
------- = ----------------------
(2)
-
-
dt
L
The current through the inductance is given by :
(
V
i
–
V
o
)
I
L
= ----------------------
(3)
-
L
When V
i
, V
o
, and L are constant, I
L
varies linearly with t. Therefore, it follows that :
(
V
i
–
V
o
)T
O N
+
-
∆
I
L
= ----------------------------------
(4)
L
When Q is OFF the current through the coil has reached its maximum value, Ipeak and because it cannot
very instantaneously, the voltage across the ased to allow the recirculation of the current through the load.
When Q switches OFF, the following situation is present:
v
C
(t) = V
o
, i
L
(t) = i
D
(t) = I
peak
And the equation associated to the following loop may be written :
d
IL
-
V
F
+
L
------- +
v
C
=
0 (5)
dt
where : v
C
= V
o
dI
L
(
V
F
+
V
o
)
------- = – ------------------------
(6)
-
-
dt
L
It follows therefore that :
V
F
+
V
o
-
i
L
(
t
)
= – -------------------
t (7)
T
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