Rainbow Electronics ATA6834 - Manual (Page 9)

9
9122B–AUTO–10/08
ATA6833/ATA6834 [Preliminary]
Example:
Using an external resistor R
WD
= 33 k
Ω
±1% results in typical parameters as follows:
T
OSC
= 12.32 µs
t
1
= 980
×
T
OSC
= 12.07 ms ±10%
t
2
= 780
×
T
OSC
= 9.609 ms ±10%
t
1
+ t
2
= 21.68 ms ±10%
Hence, the minimum time the external microcontroller has to wait before pin WD can be trig-
gered is in worst case t
min
= 1.1
×
t
1
= 13.28 ms. The maximum time for the watchdog trigger on
WD pin is t
max
= 0.9
×
(t
1
+ t
2
) = 19.51 ms. Thus watchdog trigger input must remain within
t
max
– t
min
= 6.23 ms.
Other values can be set up by picking a different resistor value for R
WD
. The dependency of T
OSC
on the value of R
WD
is shown in Figure 3-5 .
Figure 3-5. T
OSC
versus R
WD
3.5 Charge Pump
A charge pump has been implemented in order to provide sufficient voltage to operate the exter-
nal high-side power-NMOS transistors and the VG regulator, which drives the low-side
Power-NMOS transistors. The charge pump output voltage at CPOUT pin is controlled to settle
typically about 15V above the voltage at pin PBAT. A built-in supervisor circuit checks if the out-
put voltage is sufficient to operate the VG regulator and external Power-NMOS transistors. The
output voltage is accepted as good when it rises above VCP
CPGOOD
. A charge pump failure is
flagged at DG2 if this minimum can not be reached or if the output voltage drops below the lower
threshold of VCP
CPGOOD
due to overloading.
The two shuffle capacitors should have the same value. The value of the reservoir capacitor
should be at least twice the value of one shuffle capacitor. Two external shuffle capacitors and
an external reservoir capacitor have to be provided. The typical values for the two shuffle capac-
itor is 100F, and for the reservoir capacitor is 470 nF. All capacitors should be ceramic. It is
advisable to pick a reservoir capacitor with twice or three-times the size of the two equally-sized
shuffle capacitors. The greater the capacitors, the greater the output current capability.
R
WD
(k Ω )
T
OSC
(µs)
T
OSC
(µs)
0
5
10
15
2 0
2 5
3 0
3 5
4 0
4 5
10 2 0 3 0 4 0 50 6 0 70 8 0 9 0 10 0
T
OSCmin
(µs)
T
OSCmax
(µs)