LM34927SD Datasheet, PDF (11/26 Pages) Texas Instruments – LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters

LM34927
www.ti.com
SNVS799F – APRIL 2012 – REVISED DECEMBER 2013
Under Voltage Detector
The LM34927 contains a dual level Undervoltage Lockout (UVLO) circuit. When the UVLO pin voltage is below
0.66 V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.66 V but
less than 1.225 V, the controller is in standby mode. In standby mode the V_CC bias regulator is active while the
regulator output is disabled. When the V_CC pin exceeds the V_CC undervoltage thresholds and the UVLO pin
voltage is greater than 1.225V, normal operation begins. An external set-point voltage divider from V_IN to GND
can be used to set the minimum operating voltage of the regulator.
UVLO hysteresis is accomplished with an internal 20 μA current source that is switched on or off into the
impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is activated to
quickly raise the voltage at the UVLO pin. The hysteresis is equal to the value of this current times the resistance
R_UV2.
UVLO
<0.66 V
0.66 V-1.225 V
>1.225 V
Table 2. Under Voltage Detector
V_CC
V_CC < 4.5 V
V_CC > 4.5 V
Mode
Shutdown
Standby
Standby
Operating
Description
V_CC regulator disabled.
Switcher disabled.
V_CC regulator enabled.
Switcher disabled.
V_CC regulator enabled.
Switcher disabled.
V_CC enabled.
Switcher enabled.
If the UVLO pin is wired directly to the V_IN pin, the regulator will begin operation once the V_CC undervoltage is
satisfied.
VIN
+
C_IN
R_UV2
2
V_IN
LM34927
3 _UVLO
R_UV1
Figure 15. UVLO Resistor Setting
Thermal Protection
The LM34927 should be operated so the junction temperature does not exceed 150°C during normal operation.
An internal Thermal Shutdown circuit is provided to protect the LM34927 in the event of a higher than normal
junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state,
disabling the buck switch and the V_CC regulator. This feature prevents catastrophic failures from accidental
device overheating. When the junction temperature reduces below 145°C (typical hysteresis = 20°C), the V_CC
regulator is enabled, and normal operation is resumed.
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