LMZ23608 Datasheet, PDF (15/26 Pages) Texas Instruments – 8A SIMPLE SWITCHER® Power Module with 36V Maximum Input Voltage and Current Sharing

General Description
The LMZ23608 SIMPLE SWITCHER© power module is an
easy-to-use step-down DC-DC solution capable of driving up
to 8A load. The LMZ23608 is available in an innovative pack-
age that enhances thermal performance and allows for hand
or machine soldering.
The LMZ23608 can accept an input voltage rail between 6V
and 36V and deliver an adjustable and highly accurate output
voltage as low as 0.8V. The LMZ23608 only requires two ex-
ternal resistors and three external capacitors to complete the
power solution. The LMZ23608 is a reliable and robust design
with the following protection features: thermal shutdown, in-
put under-voltage lockout, output over-voltage protection,
short-circuit protection, output current limit, and allows startup
into a pre-biased output. The sync input allows synchroniza-
tion over the 350 to 600 kHz switching frequency range.
Design Steps for the LMZ23608
Application
The LMZ23608 is fully supported by Webench® which offers:
component selection, electrical and thermal simulations. Ad-
ditionally, there are both evaluation and demonstration
boards that may be used as a starting point for design. The
following list of steps can be used to manually design the
LMZ23608 application.
All references to values refer to the typical applications
schematic Figure 5 .
• Select minimum operating V_IN with enable divider resistors
• Program V_OUT with FB resistor divider selection
• Select C_OUT
• Select C_IN
• Determine module power dissipation
• Layout PCB for required thermal performance
ENABLE DIVIDER, R_ENT, R_ENB AND R_ENHSELECTION
Internal to the module is a 2 mega ohm pull-up resistor con-
nected 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 se-
quencing supplies, the divider is connected to a rail that
becomes active earlier in the power-up cycle than the
LMZ23608 output rail.
Enable provides a precise 1.274V threshold to allow direct
logic drive or connection to a voltage divider from a higher
enable voltage such as V_IN. Additionally there is 13 μA (typ)
of switched offset current allowing programmable hysteresis.
See Figure 1.
The function of the enable divider is to allow the designer to
choose an input voltage below which the circuit will be dis-
abled. This implements the feature of a programmable UVLO.
The two resistors should be chosen based on the following
ratio:
R_ENT / R_ENB = (V_{IN UVLO} / 1.274V) – 1
(1)
The LMZ23608 typical application shows 12.7kΩ for R_ENB and
42.2kΩ for R_ENT resulting in a rising UVLO of 5.51V. Note that
this divider presents 4.62V to the EN input when V_IN is raised
to 20V. This upper voltage should always be checked, making
sure that it never exceeds the Abs Max 5.5V limit for Enable.
A 5.1V Zener clamp can be applied in cases where the upper
voltage would exceed the EN input's 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 is 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) = 1.274 ( 1 + (R_ENT|| 2 meg)/ R_ENB)
(2)
Whereas the falling threshold level can be calculated using:
V_EN(falling) = V_EN(rising) – 13 µA ( R_ENT|| 2 meg ||
R_ENTB + R_ENH )
(3)
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FIGURE 1. Enable input detail
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