Texas Instruments Evaluation Board for the LM25118 Wide Vin Buck-Boost Controller LM25118EVAL/NOPB LM25118EVAL/NOPB Datenbogen
Produktcode
LM25118EVAL/NOPB
Powering Up
7
Powering Up
Connecting the IC’s enable pin to ground will allow powering up the source supply with a minimal output
load. Set the current limit of the source supply to provide about 1.5 times the anticipated wattage of the
load. Note that input currents become very high at low input voltages, which requires an appropriate input
supply. As you remove the connection from the enable pin to ground, immediately check for 12 V at the
output.
load. Set the current limit of the source supply to provide about 1.5 times the anticipated wattage of the
load. Note that input currents become very high at low input voltages, which requires an appropriate input
supply. As you remove the connection from the enable pin to ground, immediately check for 12 V at the
output.
A quick efficiency check is the best way to confirm that everything is operating properly. If something is
amiss, you can be reasonable sure that it will affect the efficiency adversely. Few parameters can be
incorrect in a switching power supply without creating losses and potentially damaging heat.
amiss, you can be reasonable sure that it will affect the efficiency adversely. Few parameters can be
incorrect in a switching power supply without creating losses and potentially damaging heat.
8
Over Current Protection
The evaluation board is configured with over-current protection. The output current is limited to
approximately 4.5 A in the buck-boost mode The 4.5 A value allows for component tolerances to specify a
3 A output current. Note this current will be almost double, or about 7 A in buck mode (VIN greater than
17 V) due to the difference in peak inductor currents in the two different modes.
approximately 4.5 A in the buck-boost mode The 4.5 A value allows for component tolerances to specify a
3 A output current. Note this current will be almost double, or about 7 A in buck mode (VIN greater than
17 V) due to the difference in peak inductor currents in the two different modes.
Figure 3. Short Circuit Current
9
VCCX
A place for a jumper between VOUT and VCCX is provided on the PC board. If operation below about 5 V
is required, connect the jumper to allow VCCX to power the converter (the exact voltage depends on the
gate drive requirements of the switching FETs). The converter does require a minimum VIN of 5 V to
initially start. When running, the input voltage can decrease to below 5 V at reduced current with VCCX
connected to VOUT. Note that this design uses a current limit value to specify a full 3 A of output current
at a minimum V
is required, connect the jumper to allow VCCX to power the converter (the exact voltage depends on the
gate drive requirements of the switching FETs). The converter does require a minimum VIN of 5 V to
initially start. When running, the input voltage can decrease to below 5 V at reduced current with VCCX
connected to VOUT. Note that this design uses a current limit value to specify a full 3 A of output current
at a minimum V
IN
of 5 V. For operation lower than 5 V, the current limit resistor, UVLO threshold, and
ramp capacitor must be re-calculated. Caution: make sure the input supply can source the required input
current. Operation at low V
current. Operation at low V
IN
at full power may overheat and damage the MOSFET’s and Diodes supplied
on the board. Note there is a limit of 14 V applied to VCCX. Never exceed this value if operating VCCX
from an external source, or operating the board with V
from an external source, or operating the board with V
OUT
greater than 12 V. To prevent oscillation,
connect and additional 100 uF or greater electrolytic capacitor across V
IN
for input voltages less than 5 V.
10
Mode Transition
With V
OUT
set at 12 V, the LM25118 applications board will operate in the buck mode with VIN greater than
about 17 V. As VIN is reduced below 17 V, the converter begins to operate in a soft buck-boost mode. As
VIN is decreased below 14 V, the converter smoothly transitions to a pure buck-boost mode. This method
of mode transition insures a smooth, glitch free operation as VIN is varied over the transition region.
VIN is decreased below 14 V, the converter smoothly transitions to a pure buck-boost mode. This method
of mode transition insures a smooth, glitch free operation as VIN is varied over the transition region.
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AN-2205 LM25118 Evaluation Board
SNVA614B – January 2012 – Revised January 2014
Copyright © 2012–2014, Texas Instruments Incorporated