Datasheet

ENABLE
2.0M
21 uA
INT-VCC (5V)
1.279V
RUN
RENB
12.7k
RENT
42.2k
VIN
100Ö
RENH
5.1V
P/O LMZ22005
LMZ22005
www.ti.com SNVS686I –MARCH 2011–REVISED OCTOBER 2013
DESIGN STEPS FOR THE LMZ22005 APPLICATION
The LMZ22005 is fully supported by Webench® which offers: component selection, electrical and thermal
simulations. Additionally there is are evaluation and demonstration boards that may be used a starting point for
design. The following list of steps can be used to quickly design the LMZ22005 application.
• Select minimum operating V
IN
with enable divider resistors
• Program V
O
with resistor divider selection
• Select C
O
• Select C
IN
• Determine module power dissipation
• Layout PCB for required thermal performance
ENABLE DIVIDER, R
ENT
, R
ENB
AND R
ENH
SELECTION
Internal to the module is a 2 mega ohm pull-up resistor connected from V
IN
to Enable. For applications not
requiring precision under voltage lock out (UVLO), the Enable input may be left open circuit and the internal
resistor will always enable the module. In such case, the internal UVLO occurs typically at 4.3V (V
IN
rising).
In applications with separate supervisory circuits Enable can be directly interfaced to a logic source. In the case
of sequencing supplies, the divider is connected to a rail that becomes active earlier in the power-up cycle than
the LMZ22005 output rail.
Enable provides a precise 1.279V threshold to allow direct logic drive or connection to a voltage divider from a
higher enable voltage such as V
IN
. Additionally there is 21 μA(typ) of switched offset current allowing
programmable hysteresis. See Figure 46.
The function of the enable divider is to allow the designer to choose an input voltage below which the circuit will
be disabled. This implements the feature of programmable UVLO. The two resistors should be chosen based on
the following ratio:
R
ENT
/ R
ENB
= (V
IN UVLO
/ 1.279V) – 1 (1)
The LMZ22005 typical application shows 12.7kΩ for R
ENB
and 42.2kΩ for R
ENT
resulting in a rising UVLO of
5.46V. Note that a midpoint 5.1V Zener clamp is present to allow setting UVLO to cover an extended range of
operation. The zener clamp is not required if the target application prohibits the maximum Enable input voltage
from being exceeded.
Additional enable voltage hysteresis can be added with the inclusion of R
ENH
. It may be possible to select values
for R
ENT
and R
ENB
such that R
ENH
is a value of zero allowing it to be omitted from the design.
Rising threshold can be calculated as follows:
V
EN
(rising) = 1.279 ( 1 + R
ENT
|| 2 meg/ R
ENB
)
Whereas falling threshold level can be calculated using:
V
EN
(falling) = V
EN
(rising) – 21 µA ( R
ENT
|| 2 meg || R
ENTB
+ R
ENH
)
Figure 46. Enable Input Detail
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