Datasheet

'I
I
OUT2
I
OUT3
Normal
Operation
Load Current
Decreases
Current
Limited
Load
Current
Increases
Normal
Operation
Current Limit
Threshold
Inductor Current
Feedback
Voltage
@ FB Pin
I
OUT1
T
ON
2.5V
T
ON
2
LM34914
www.ti.com SNVS453B –MAY 2006–REVISED MARCH 2013
Figure 10. Inductor Current - Normal and Current Limit Operation
N - Channel Buck Switch and Driver
The LM34914 integrates an N-Channel buck switch and associated floating high voltage gate driver. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022
µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During
each off-time, the SW pin is at approximately -1V, and C4 is recharged for the next on-time from V
CC
through the
internal diode. The minimum off-time ensures a minimum time each cycle to recharge the bootstrap capacitor.
Softstart
The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing
start-up stresses and current surges. Upon turn-on, after V
CC
reaches the under-voltage threshold, an internal
12.5 µA current source charges up the external capacitor at the SS pin to 2.5V (t
2
in Figure 8). The ramping
voltage at SS (and the non-inverting input of the regulation comparator) ramps up the output voltage in a
controlled manner.
An internal switch grounds the SS pin if V
CC
is below the under-voltage lockout threshold, or if the RON/SD pin is
grounded.
Thermal Shutdown
The LM34914 should be operated so the junction temperature does not exceed 125°C. If the junction
temperature increases above that, an internal Thermal Shutdown circuit activates (typically) at 175°C, taking the
controller to a low power reset state by disabling the buck switch and the on-timer. This feature helps prevent
catastrophic failures from accidental device overheating. When the junction temperature reduces below 155°C
(typical hysteresis = 20°C), normal operation resumes.
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