Datasheet

LMR24210
www.ti.com SNVS738G –OCTOBER 2011–REVISED APRIL 2013
N-Channel MOSFET and Driver
The LMR24210 integrates an N-Channel main MOSFET and an associated floating high voltage main MOSFET
gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor C
BST
and an internal
high voltage diode. C
BST
connecting between the BST and SW pins powers the main MOSFET gate driver during
the main MOSFET on-time. During each off-time, the voltage of the SW pin falls to approximately -1V, and C
BST
charges from V
CC
through the internal diode. The minimum off-time of 260 ns provides enough time for charging
C
BST
in each cycle.
Soft-Start
The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing
startup stresses and current surges. Upon turn-on, after V
CC
reaches the under-voltage threshold, an 8 µA
internal current source charges up an external capacitor C
SS
connecting to the SS pin. The ramping voltage at
the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the output voltage V
OUT
in
a controlled manner.
The soft start time duration to reach steady state operation is given by the formula:
t
SS
= V
REF
x C
SS
/ 8µA = 0.8V x C
SS
/ 8µA (7)
This equation can be rearranged as follows:
C
SS
= t
SS
x 8µA / 0.8V (8)
Use of a 4.7nF capacitor results in a 0.5ms soft-start duration. This is a recommended value. Note that high
values of C
SS
capacitance will cause more output voltage drop when a load transient goes across the DCM-CCM
boundary. If a fast load transient response is desired for steps between DCM and CCM mode the softstart
capacitor value should be less than 18nF (which corresponds to a soft-start time of 1.8ms).
An internal switch grounds the SS pin if any of the following three cases happens: (i) V
CC
is below the under-
voltage lock-out threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the output
voltage can be shut off by connecting the SS pin to ground using an external switch. Releasing the switch allows
the SS pin to ramp up and the output voltage to return to normal. The shutdown configuration is shown in
Figure 22.
Figure 22. Alternate Shutdown Implementation
Thermal Protection
The junction temperature of the LMR24210 should not exceed the maximum limit. Thermal protection is
implemented by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller
enter a low power reset state by disabling the main MOSFET, disabling the on-timer, and grounding the SS pin.
Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction
temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation
resumes.
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LMR24210