LMR14050-Q1_16 Datasheet, PDF (11/35 Pages) Texas Instruments – LMR14050-Q1 SIMPLE SWITCHER® 40 V, 5 A Step-Down Converter with 40 μA IQ

www.ti.com
LMR14050-Q1
SNVSAG2A – NOVEMBER 2015 – REVISED JULY 2016
Feature Description (continued)
7.3.4 Low Dropout Operation and Bootstrap Voltage (BOOT)
The LMR14050-Q1 provides an integrated bootstrap voltage regulator. A small capacitor between the BOOT and
SW pins provides the gate drive voltage for the high-side MOSFET. The BOOT capacitor is refreshed when the
high-side MOSFET is off and the external low side diode conducts. The recommended value of the BOOT
capacitor is 0.1 μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 16 V or
greater is recommended for stable performance over temperature and voltage.
When operating with a low voltage difference from input to output, the high-side MOSFET of the LMR14050-Q1
will operate at approximate 97% duty cycle. When the high-side MOSFET is continuously on for 5 or 6 switching
cycles (5 or 6 switching cycles for frequency lower than 1 MHz, and 10 or 11 switching cycles for frequency
higher than 1 MHz) and the voltage from BOOT to SW drops below 3.2 V, the high-side MOSFET is turned off
and an integrated low side MOSFET pulls SW low to recharge the BOOT capacitor.
Since the gate drive current sourced from the BOOT capacitor is small, the high-side MOSFET can remain on for
many switching cycles before the MOSFET is turned off to refresh the capacitor. Thus the effective duty cycle of
the switching regulator can be high, approaching 97%. The effective duty cycle of the converter during dropout is
mainly influenced by the voltage drops across the power MOSFET, the inductor resistance, the low side diode
voltage and the printed circuit board resistance.
7.3.5 Adjustable Output Voltage
The internal voltage reference produces a precise 0.75 V (typical) voltage reference over the operating
temperature range. The output voltage is set by a resistor divider from output voltage to the FB pin. It is
recommended to use 1% tolerance or better and temperature coefficient of 100 ppm or less divider resistors.
Select the low side resistor R_FBB for the desired divider current and use Equation 1 to calculate high-side R_FBT.
Larger value divider resistors are good for efficiency at light load. However, if the values are too high, the
regulator will be more susceptible to noise and voltage errors from the FB input current may become noticeable.
R_FBB in the range from 10 kΩ to 100 kΩ is recommended for most applications.
V_OUT
R_FBT
FB
R_FBB
Figure 14. Output Voltage Setting
R_FBT
V_OUT 0.75
0.75
u
R_FBB
(1)
7.3.6 Enable and Adjustable Under-voltage Lockout
The LMR14050-Q1 is enabled when the VIN pin voltage rises above 3.7 V (typical) and the EN pin voltage
exceeds the enable threshold of 1.2 V (typical). The LMR14050-Q1 is disabled when the VIN pin voltage falls
below 3.42 V (typical) or when the EN pin voltage is below 1.2 V. The EN pin has an internal pull-up current
source (typically I_EN = 1 μA) that enables operation of the LMR14050-Q1 when the EN pin is floating.
Many applications will benefit from the employment of an enable divider R_ENT and R_ENB in Figure 13 to establish
a precision system UVLO level for the stage. System UVLO can be used for supplies operating from utility power
as well as battery power. It can be used for sequencing, ensuring reliable operation, or supply protection, such
as a battery. An external logic signal can also be used to drive EN input for system sequencing and protection.
When EN terminal voltage exceeds 1.2 V, an additional hysteresis current (typically I_HYS = 3.6 μA) is sourced out
of EN terminal. When the EN terminal is pulled below 1.2 V, I_HYS current is removed. This additional current
facilitates adjustable input voltage UVLO hysteresis. Use Equation 2 and Equation 3
Equation 3 to calculate R_ENT and R_ENB for desired UVLO hysteresis voltage.
Copyright © 2015–2016, Texas Instruments Incorporated
Product Folder Links: LMR14050-Q1
Submit Documentation Feedback
11