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LM21215AMHE-1 Datasheet(PDF) 15 Page - Texas Instruments
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LM21215AMHE-1 Datasheet(HTML) 15 Page - Texas Instruments
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SNOSB87C – MARCH 2011 – REVISED JANUARY 2016
Feature Description (continued)
7.3.7 Short Circuit Protection
In an event that the output is shorted with a low impedance to ground, the LM21215A limits the current into the
short by resetting the device. A short circuit condition is sensed as a current limit condition coinciding with a
voltage on the FB pin that is lower than 100 mV. When this condition occurs, the device begins its reset
sequence, turning off both power MOSFETs and discharging the soft-start capacitor after t
µs). The device then attempts to restart. If the short circuit condition still exists, it resets again, repeating until the
short circuit condition is cleared. The reset prevents excessive power MOSFET dissipation and limits thermal
stress during a short circuit fault condition.
7.4 Device Functional Modes
7.4.1 Light-Load Operation
The LM21215A maintains high efficiency when operating at light loads. Whenever the load current is reduced to
a level less than half the peak-to-peak inductor ripple current, the device enters discontinuous conduction mode
(DCM) and prevents negative inductor current. The low-side MOSFET then operates in diode emulation mode
(DEM), conducting only positive inductor current. Calculate the critical conduction boundary using Equation 2.
Several diagrams are shown in Figure 27 illustrating continuous conduction mode (CCM), discontinuous
conduction mode (DCM), and the boundary condition.
When the inductor current reaches zero, the SW node becomes high impedance. Resonant ringing occurs at SW
as a result of the LC tank circuit formed by the filter inductor and the parasitic capacitance at the SW node. At
very light loads, usually below 500 mA, several pulses may be skipped in between switching cycles, effectively
reducing the switching frequency and further improving light-load efficiency.
7.4.2 Overvoltage and Undervoltage Handling
The LM21215A has built-in undervoltage protection (UVP) and overvoltage protection (OVP) using FB voltage
comparators to control the power MOSFETs. The rising OVP threshold is typically set at 112.5% of the nominal
voltage setpoint. Whenever excursions occur in the output voltage above the OVP threshold, the device
terminates the present on-pulse, turns on the low-side MOSFET, and pulls PGOOD low. The low-side MOSFET
remains on until either the FB voltage falls back into regulation or the inductor current zero-cross is detected. If
the output reaches the falling UVP threshold, typically 90% of the nominal setpoint, the device continues
switching and PGOOD is asserted and pulls low. As detailed in the PGOOD Indicator section, PGOOD has 15
of built-in deglitch time in response to a UVP or OVP condition to avoid false tripping during transient glitches.
7.4.3 Thermal Shutdown
Internal thermal shutdown circuitry is provided to protect the integrated circuit in the event that the maximum
junction temperature is exceeded. When activated, typically at 165°C, the LM21215A tri-states the power
MOSFETs and resets soft start. After the junction temperature cools to approximately 155°C, the device starts up
using the normal startup routine.
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