LM34930 Datasheet, PDF (11/24 Pages) National Semiconductor (TI) – Ultra Small 33V, 1A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

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LM34930
SNVS571B – MAY 2008 – REVISED APRIL 2013
I_PK
I_OCL
Current
Limit
Threshold
Io1
2.5V
Io2
'I
Inductor Current
Voltage at FB Pin
Normal Operation
Load
Current
Increases
Current Limited
Load
Current
Decreases
Normal
Operation
Figure 18. Inductor Current - Normal and Current Limit Operation
Startup Regulator, V_CC
The startup bias regulator is integral to the LM34930. The input pin (VIN) can be connected directly to the main
power source, and has transient capability to 44V. The VCC output is regulated at 7.0V, and is current limited to
approximately 15 mA. Upon power up, the regulator sources current into the external capacitor at VCC. When
the voltage on the VCC pin reaches the under-voltage lock-out (UVLO) threshold, the buck switch is enabled and
the Soft-start pin is released to allow the Soft-start capacitor to charge. The minimum input voltage is determined
by the regulator’s dropout voltage, the VCC UVLO falling threshold, and the switching frequency. When VCC falls
below the falling threshold the V_CC UVLO activates to shut off the buck switch.
Over-Voltage Indicator
The nOV pin, an open drain logic output, switches low when the voltage at VIN exceeds 19V. The over-voltage
indicator comparator provides 1.95V hysteresis to reject noise and ripple on the VIN pin. A pull-up resistor is
required at the nOV output pin to a voltage that does not exceed 7 volts. The pull-up voltage can exceed the
voltage at VIN. When nOV is low, the current into the pin must not exceed 10 mA.
Input Over-Voltage Shutdown
If the input voltage at VIN increases above 36V an internal comparator disables the buck switch, and grounds the
soft-start pin. The over-voltage shutdown comparator provides 400 mV hysteresis to reject noise and ripple on
the VIN pin. Normal operation resumes when the voltage at VIN is reduced below the lower threshold.
N-Channel Buck Switch and Driver
The LM34930 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 from V_CC through the internal diode. The
minimum off-time ensures a sufficient time each cycle to recharge the bootstrap capacitor.
Copyright © 2008–2013, Texas Instruments Incorporated
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