LM34927SD Datasheet, PDF (10/26 Pages) Texas Instruments – LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters

LM34927
SNVS799F – APRIL 2012 – REVISED DECEMBER 2013
www.ti.com
Figure 14. T_ON vs V_IN and R_ON
Current Limit
The LM34927 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 1.02 A the
present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is
controlled by the FB voltage and the input voltage V_IN. As an example, when FB = 0 V and V_IN = 48 V, a
maximum off-time is set to 16 μs. This condition occurs when the output is shorted, and during the initial part of
start-up. This amount of time ensures safe short circuit operation even up to the maximum input voltage of 100
V.
In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is
reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and
start-up time. The off-time is calculated from Equation 5:
T_OFF(ILIM)
=
0.07 ´ V_IN
V_FB + 0.2 V
ms
(5)
The current limit protection feature is peak limited, the maximum average output will be less than the peak.
N-Channel Buck Switch and Driver
The LM34927 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 0.01
uF ceramic capacitor connected between the BST pin and SW pin provides the voltage to the driver during the
on-time. During each off-time, the SW pin is at approximately 0 V, and the bootstrap capacitor charges from V_CC
through the internal diode. The minimum off-timer, set to 144 ns, ensures a minimum time each cycle to recharge
the bootstrap capacitor.
Synchronous Rectifier
The LM34927 provides an internal synchronous N-Channel MOSFET rectifier. This MOSFET provides a path for
the inductor current to flow when the high-side MOSFET is turned off.
The synchronous rectifier has no diode emulation mode, and is designed to keep the regulator in continuous
conduction mode even during light loads which would otherwise result in discontinuous operation. This feature
specifically allows the user to design a secondary regulator using a transformer winding off the main inductor to
generate the alternate regulated output voltage.
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