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DC/DC Converter Section
AN1290
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STMicroelectronics Confidential
7
DC/DC Converter Section
7.1
Structure of the DC/DC Converter and B+ Loop
7.1.1
Structure of the Converter Section
The DC/DC converter section has a basic structure very similar to the well-known UC 3842 family,
i.e. current-mode PWM converters. (You can refer to the corresponding datasheet). It includes:
●
An oscillator to switch the power MOS transistor ON. In fact the horizontal oscillator is used for
this purpose, ensuring correct synchronization. Triggering can take place at various points of
the period selected through I²C programming (Sad07h/d7 and Sad17h/d3), in order to
minimize the disturbance by switching parasitic voltages.
●
An error amplifier, which compares a fraction of the voltage to be regulated with an internal
voltage reference, and amplifies the difference.
The internal reference is approximately 4.8 V (rather than 2.5V as in UC 3842). It may be
adjusted by ±20% through I²C (Register 03), except on the TDA9115.
●
An (Isense) pin (pin 16) which measures the current in the switch, or rather the voltage drop on
a low-valued series current-sense resistor
●
A comparator to switch OFF the power transistor when (Isense) voltage reaches a fixed
fraction (1/3) of the amplified error voltage. For safety, switching OFF will take place anyway if
the voltage on the Isense pin reaches 2V. With the TDA9112A, you can choose by I²C
programming (Sad01Fh/d7) between 2V and 1.2V for switching OFF. (1.2V is best suited for
normal applications, 2V for open-loop converters.)
The duty factor is not limited and can almost reach 100%. (Nevertheless, after switching OFF, the
power MOS transistor will remain OFF for approximately 300 ns.)
Compared to older circuits like the TDA9111, the DC/DC section has been slightly modified to allow
new applications, like Open-loop control.
7.1.2
External or Internal Sawtooth Configuration
The above description corresponds to the Current mode, step-up configuration (or to the related
configuration: Current mode, Step-down) and makes use of the external sawtooth of pin 16.
It has distinctive advantages:
●
very simple control of the power MOS transistor,
●
pulse-by-pulse current limitation,
●
loop stability is easily guaranteed because the converter itself has a first-order frequency
response (as long as operation is in discontinuous conduction mode, i.e. current in the rectifier
falls to 0 at each period),
●
parasitic voltages generated only at switch-OFF (in discontinuous mode).
On the other hand, remembering that switching is synchronized to the Horizontal section (i.e. a
complete cycle must take place within one H period), a large frequency range may be obtained only
at the expense of higher peak current. This means a larger power MOS transistor.
With the same circuit, it is also possible to implement a step-down, continuous conduction
converter. In this case, a high-side switch is used. Continuous conduction converters are generally
not current-mode, but voltage-mode. The error amplifier output is no longer compared to an image
of the current, but to any sawtooth.
