ATA5279 ATMEL Corporation, ATA5279 Datasheet - Page 7

• Manufacturer Part Number: ATA5279
• Manufacturer: ATMEL Corporation

3.4
9125A–RKE–04/08
Boost Converter
The coil driver stage can be supplied by a DC-DC converter in boost configuration. Together
with an externally applied choke, freewheeling diode and capacitor, the battery voltage can be
brought up to the required value, which is dependant on the coil's impedance and the selected
current. The converter is only enabled during an active transmission. To prevent damage from
the IC or the external circuitry, the peak current through the low-side switch is limited and opera-
tion is shut down if the output voltage exceeds an upper limit.
The switching frequency is, like the coil driving signal, derived from the oscillator stage and
125 kHz in value when using an 8 MHz input clock. To improve EMC behavior, the switching
time of the integrated low-side switch is chosen to give the best compromise between power dis-
sipation and low dV/dt’s.
The least possible time the boost converter takes to generate the maximum possible output volt-
age from the minimum possible input voltage is dependant on several parameters. The
dimensions of the external components (choke inductance, charge capacitance) greatly effects
this time. For the suggested values (see
shows the voltage-over-time plot:
Figure 3-3.
Note that for this plot, the load on the VDS line corresponds to an active coil operated at
700 mAp. The IC is expected to be in idle mode in order to reach the speeds shown above, as a
wake-up from power-down takes additional time. Generally, the speed depends on the value of
the external integration capacitor at the CINT pin. This capacitance should be large enough to
ensure a stable voltage regulation.
Worst case occurs at high junction temperatures (T
of 7.5V. Typically, the junction temperature is 30 °C and the supply voltage is > 10.5V.
Boost Converter Maximum Speed (Worst Case and Typical)
45
40
35
30
25
20
20
20
20
0
0
Typical
0.0001
0.0002
Section 4. “Application” on page
0.0003
t (s)
0.0004
j
= 150°C) and a supply voltage at the VS pin
ATA5279 [Preliminary]
0.0005
Worst case
0.0006
0.0007
27). The figure below
7