TEA1205 Philips Semiconductors, TEA1205 Datasheet - Page 5

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TEA1205

Manufacturer Part Number
TEA1205
Description
High efficiency DC/DC converter
Manufacturer
Philips Semiconductors
Datasheet

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PINNING
FUNCTIONAL DESCRIPTION
Control mechanism
The TEA1205AT DC/DC converter is able to operate in
discontinuous or continuous conduction operation.
All switching actions are completely determined by a
digital control circuit which uses the output voltage level as
its control input. This novel digital approach enables the
use of a new pulse width and frequency modulation
scheme, which ensures optimum power efficiency over the
complete range of operation of the converter. The scheme
works as follows. At low output power, a very small current
pulse is generated in the inductor, and the pulse rate
varies with a varying load. When the output voltage drops
below a specific limit, which indicates that the converter’s
current capability is not sufficient, the digital controller
switches to the next state of operation. The peak current in
the inductor is made higher, and the pulse rate can again
vary with a varying load. A third operation state is available
for again higher currents.
When high output power is requested, the device starts
operating in continuous conduction mode. This results in
minimum AC currents in the circuit components and hence
optimum efficiency, cost, and EMC. In this mode, the
output voltage is allowed to vary between two predefined
voltage levels. As long as the output voltage stays within
this so-called window, switching continues in a fixed
pattern. When the output voltage reaches one of the
window borders, the digital controller immediately reacts
by adjusting the pulse width and inserting a current step in
such a way that the output voltage stays within the window
with higher or lower current capability. This approach
enables very fast reaction to load variations. Figure 3
shows the various coil current waveforms for low and high
current capability in each power conversion mode.
Figure 4 shows the converter’s response to a sudden load
increase. The upper trace shows the output voltage.
1998 Mar 24
VSEL
SYNC
OUT
SENSE
LX
GND
ILIM
SHDWN
High efficiency DC/DC converter
SYMBOL
PIN
1
2
3
4
5
6
7
8
output voltage selection input
synchronisation clock input
output voltage output
output voltage sense input
inductor connection
ground
current limit resistor connection
shut-down input
DESCRIPTION
5
The ripple on top of the DC level is a result of the current
in the output capacitor, which changes in sign twice per
cycle, times the capacitor’s internal Equivalent Series
Resistance (ESR). After each ramp-down of the inductor
current, i.e. when the ESR effect increases the output
voltage, the converter determines what to do in the next
cycle. As soon as more load current is taken from the
output the output voltage starts to decay. When the output
voltage becomes lower than the low limit of the window,
a corrective action is taken by a ramp-up of the inductor
current during a much longer time. As a result, the DC
current level is increased and normal continuous
conduction mode can continue. The output voltage
(including ESR effect) is again within the predefined
window.
Figure 5 depicts the spread of the output voltage window.
The absolute value is most dependent on spread, while the
actual window size is not affected. For one specific device,
the output voltage will not vary more than 4%.
Start-up
A possible deadlock situation in boost configuration can
occur after a sequence of disconnecting and reconnecting
the input voltage source. If, after disconnection of the input
source, the output voltage falls below 2.0 V, the device
may not restart properly after reconnection of the input
source, and may take continuous current from the input.
An external circuit to prevent the deadlock situation is
shown in Chapter “Application information”.
Shut-down
When the shut-down pin is made HIGH, the converter
disables both switches and power consumption is reduced
to a few A.
handbook, halfpage
SENSE
SYNC
VSEL
OUT
Fig.2 Pin configuration.
1
2
3
4
TEA1205AT
MGM697
Preliminary specification
8
7
6
5
TEA1205AT
SHDWN
ILIM
GND
LX

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