LM34927SD Datasheet, PDF (9/26 Pages) Texas Instruments – LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters

LM34927
www.ti.com
SNVS799F – APRIL 2012 – REVISED DECEMBER 2013
At high input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall
achievable output power. As an example, with the input at 48 V and switching at high frequency, the V_CC
regulator may supply up to 7 mA of current resulting in 48 V x 7 mA = 336 mW of power dissipation. If the V_CC
voltage is driven externally by an alternate voltage source, between 8 V and 14 V, the internal regulator is
disabled. This reduces the power dissipation in the IC.
SW
LM34927
FB
L1
R_FB2
R_FB1
R_C
+
COUT
VOUT
VOUT
(low ripple)
Figure 13. Low Ripple Output Configuration
Regulation Comparator
The feedback voltage at FB is compared to an internal 1.225 V reference. In normal operation, when the output
voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225 V. The high side
switch will stay on for the on-time, causing the FB voltage to rise above 1.225 V. After the on-time period, the
high side switch will stay off until the FB voltage again falls below 1.225 V. During start-up, the FB voltage will be
below 1.225 V at the end of each on-time causing the high side switch to turn on immediately after the minimum
forced off-time of 144 ns. The high side switch can be turned off before the on-time is over, if peak current in the
inductor reaches the current limit threshold.
Overvoltage Comparator
The feedback voltage at FB is compared to an internal 1.62 V reference. If the voltage at FB rises above 1.62 V
the on-time pulse is immediately terminated. This condition can occur if the input voltage and/or the output load
changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225 V.
On-Time Generator
The on-time for the LM34927 is determined by the R_ON resistor, and is inversely proportional to the input voltage
(V_IN), resulting in a nearly constant frequency as V_IN is varied over its range. The on-time equation for the
LM34927 is:
T_ON
=
10^-10 x R_ON
V_IN
(4)
See Figure 14. R_ON should be selected for a minimum on-time (at maximum V_IN) greater than 100 ns, for proper
operation. This requirement limits the maximum frequency for each application.
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