Page 24 ams Datasheet
Document Feedback [v2-23] 2016-Mar-30
AS3685A/AS3685B − Detailed Description
AS3685A and AS3685B TXMask Function
If the battery has to supply two high currents at the same time
(e.g. the AS3685 flash and a RF-power amplifier) it is possible,
that the total current causes a high voltage drop on the battery
resulting in a shutdown of the complete system. In order to
avoid this shutdown, the AS3685 (AS3685A or AS3685B) can
reduce its current with the signal ‘TXMask’ using the following
circuit:
Figure 36:
TXMask Function of the AS3685
The TXMask signal is connected to e.g. the (RF-) power amplifier
enable pin (active high if the PA is enabled). This reduces the
flash current if the power amplifier is enabled and avoids the
unexpected shutdown of the system.
Note(s): The internal flash timeout timer (t
FLASHTIMEOUT
) to limit
the total flash duration, is not affected by the TXMask function
(see also section Flash Timeout).
The I
ISET
current (current into the pin ISET) for TXMask = 0 can
be calculated with:
For TXMask = 1 the current I
ISET
is reduced to:
V(TXMask) is the actual voltage for the high level (‘1’) for the
signal TXMask
ISET
VBAT
EN1
EN2
VBAT
EN1
EN2
RISET
RTXMASK
I
ISET TXMASK, 0=
V
ISET
R
ISET
---------------
V
ISET
R
TXMASK
-------------------------
+
1.3V
R
ISET
-------------- -
1.3V
R
TXMASK
-------------------------
+==
I
ISET TXMASK, 1=
V
ISET
R
ISET
------------------
V
ISET
V TXMASK()–
R
TXMASK
-------------------------------------------------------------
+
1.3V
R
ISET
----------------- -
1.3 V V T XM ASK()–
R
TXMASK
-------------------------------------------------------
+==
