LM3880_14 Datasheet, PDF (8/20 Pages) Texas Instruments – Power Sequencer

LM3880, LM3880Q
SNVS451H – AUGUST 2006 – REVISED JULY 2011
www.ti.com
The timing sequence of the LM3880 is controlled entirely by the enable (EN) pin. Upon power up, all the flags
are held low until this precision enable is pulled high. After the EN pin is asserted, the power up sequence will
commence. An internal counter will delay the first flag (FLAG1) from rising until a fixed time period has expired.
Upon the release of the first flag another timer will begin to delay the release of the second flag (FLAG2). This
process repeats until all three flags have sequentially been released. The three timers that control the delays are
all independent of each other and can be individually programmed if needed. (See custom sequencer section).
The power down sequence is the same as power-up, but in reverse. When EN pin is de-asserted a timer will
begin that delays the third flag (FLAG3) from pulling low. The second and first flag will then follow in a sequential
manner after their appropriate delays. The three timers that are used to control the power down scheme can also
be individually programmed and are completely independent of the power up timers.
Additional sequence patterns are also available in addition to customizable timers. For more information see the
custom sequencer section.
PART OPERATION
The timing sequence of the LM3880 is controlled by the assertion of the enable signal. The enable pin is
designed with an internal comparator, referenced to a bandgap voltage (1.25V), to provide a precision threshold.
This allows a delayed timing to be externally set using a capacitor or to start the sequencing based on a certain
event, such as a line voltage reaching 90% of nominal. For an additional delayed sequence from the rail
powering VCC, simply attach a capacitor to the EN pin as shown below.
7 PA
EN
+
C_EN
1.25V
-
Enable
Figure 5. Cap Timing
Using the internal pull-up current source to charge the external capacitor (C_EN) the enable pin delay can be
calculated by the equation below:
1.25V x C_EN
t_{enable_delay} =
7 PA
(1)
A resistor divider can also be used to enable the LM3880 based on a certain voltage threshold. Care needs to be
taken when sizing the resistor divider to include the effects of the internal current source.
One of the features of the enable pin is that it provides glitch free operation. The first timer will start counting at a
rising threshold, but will always reset if the enable pin is de-asserted before the first output flag is released. This
can be shown in the timing diagram below:
EN
FLAG1
t_d1
Figure 6. EN Glitch
8
Submit Documentation Feedback
Copyright © 2006–2011, Texas Instruments Incorporated
Product Folder Links: LM3880 LM3880Q