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LM21215AMHX-1/NOPB Datasheet(PDF) 12 Page - Texas Instruments

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Part No. LM21215AMHX-1/NOPB
Description  15-A Ultra High-Efficiency Synchronous Buck Converter
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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LM21215AMHX-1/NOPB Datasheet(HTML) 12 Page - Texas Instruments

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VOUT
VEN
Time
VPGOOD
Soft-Start Time, tSS
VUVP
90% VOUT
0V
Enable Delay,
tRESETSS
SS
SS
SS
t
I
C
0.6V
˜
LM21215A
SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
www.ti.com
7.3 Feature Description
7.3.1 Precision Enable
The EN pin allows the output of the device to be enabled or disabled with an external control signal. This pin is a
precision analog input that enables the device when the voltage exceeds 1.35 V (typical). The EN pin has 110
mV of hysteresis and disables the output when the Enable voltage falls below 1.24 V (typical). If the EN pin is not
used, it can be left open as it is pulled high by an internal 2-µA current source. Since the EN pin has a precise
turn-on threshold, it can be used along with an external resistor divider network from VIN to configure the device
to turn on at a precise input voltage.
7.3.2 Input Voltage UVLO
The LM21215A has a built-in undervoltage lockout protection circuit that prevents the device from switching until
the input voltage reaches 2.7 V (typical). The UVLO threshold has 200 mV of hysteresis that prevents the device
from responding to power-on glitches during startup. As mentioned above, adjust the turn-on threshold of the
supply by using the precision enable pin and a resistor divider network connected to VIN. Please refer to
Figure 30 of the Detailed Design Procedure section for more detail.
7.3.3 Soft-Start Capability
When EN exceeds 1.35 V and AVIN is above its UVLO threshold of 2.7 V, the LM21215A begins charging the
output linearly to the voltage setpoint dictated by the feedback resistor network. The LM21215A employs a user-
adjustable soft-start circuit to set the output voltage ramp time during startup. A capacitor from SS/TRK to GND
sets the required soft-start time. Once the enable voltage exceeds 1.35 V, an internal 1.9-µA current source
begins to charge the soft-start capacitor. This allows the user to limit inrush currents due to a high output
capacitance and avoid an overcurrent condition. Adding a soft-start capacitor also reduces the stress on the input
rail. Use Equation 1 to calculate the soft-start capacitance.
where
ISS is nominally 1.9 µA
tSS is the desired startup time
(1)
If VIN is higher than the UVLO level and Enable is toggled high, the soft-start sequence begins. There is a small
delay between enable transitioning high and the beginning of the soft-start sequence. This delay allows the
LM21215A to initialize its internal circuitry. Once the output has charged to 90% of the nominal output voltage,
the PGOOD flag transitions high. This behavior is illustrated in Figure 23.
Figure 23. Soft-Start Timing
12
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