LM34910_15 Datasheet, PDF (9/18 Pages) Texas Instruments – High Voltage (40V, 1.25A) Step Down Switching Regulator

LM34910
www.ti.com
SNVS297B – OCTOBER 2004 – REVISED MARCH 2013
Figure 7 illustrates the inductor current waveform. During normal operation the load current is Io, the average of
the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches
1.25A. During the Current Limited portion of Figure 7, the current ramps down to 1.25A during each off-time,
initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an
amount equal to:
ΔI = (V_IN - V_OUT) x t_ON / L1
(7)
During this time the LM34910 is in a constant current mode, with an average load current (I_OCL) equal to 1.25A +
ΔI/2.
I_PK
'I
I_OCL
1.25A
I_O
Normal Operation
Load Current
Increases
Current Limited
Figure 7. Inductor Current - Current Limit Operation
The current limit threshold can be increased by connecting an external resistor between S_GND and I_SEN. The
external resistor will typically be less than 1Ω. The peak current out of SW and I_SEN must not exceed 3.5A. The
average current out of SW must be less than 3A, and the average current out of I_SEN must be less than 2A.
Therefore I_PK in Figure 7 must not exceed 3.5A, and I_OCL must not exceed 2A.
N - Channel Buck Switch and Driver
The LM34910 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak
current allowed through the buck switch is 3.5A, and the maximum allowed average current is 3A. 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 charges from V_CC through the internal diode. The
minimum off-time of 280 ns 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
11.5 µA current source charges up the external capacitor at the SS pin to 2.5V. 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, if a thermal shutdown
occurs, or if the R_ON/SD pin is grounded.
Thermal Shutdown
The LM34910 should be operated so the junction temperature does not exceed 125°C. If the junction
temperature increases, an internal Thermal Shutdown circuit, which activates (typically) at 175°C, takes the
controller to a low power reset state by disabling the buck switch and the on-timer, and grounding the Softstart
pin. This feature helps prevent catastrophic failures from accidental device overheating. When the junction
temperature reduces below 155°C (typical hysteresis = 20°C), the Softstart pin is released and normal operation
resumes.
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