LM34927SD Datasheet, PDF (13/26 Pages) Texas Instruments – LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters

LM34927
www.ti.com
SNVS799F – APRIL 2012 – REVISED DECEMBER 2013
Lowest Part Count Isolated Application Schematic
A low part count schematic for isolated bias application is shown in Figure 18. The primary should not be loaded
in this configuration. If primary loading is required a diode will be required between V_OUT primary and V_CC.
20 V to 100 V
VIN
C_IN ^{+ R}UV2
0.47 F ^{127 NŸ}
R_UV1
8.25 NŸ
R_ON
130 NŸ
LM34927
V_IN
RON
BST
SW
UVLO
RTN
VCC
FB
10V, 300 mA
D1
VOUT2
33 H
+ ^CBST
10 nF
X1
+ _COUT2
1 F
33 H
R_FB2
7.32 NŸ
R_FB1
1.00 NŸ
R_C
1Ÿ
+ C_OUT1
1 F
Figure 18. Lowest Part Count Isolated Application Schematic
Ripple Configuration
LM34927 uses Constant-On-Time (COT) control scheme, in which the on-time is terminated by an on-timer, and
the off-time is terminated by the feedback voltage (V_FB) falling below the reference voltage (V_REF). Therefore, for
stable operation, the feedback voltage must decrease monotonically, in phase with the inductor current during
the off-time. Furthermore this change in feedback voltage (ΔV_FB) during off-time must be large enough to
suppress any noise component present at the feedback node.
Table 3 shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1
and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output
voltage ripple has two components:
1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor.
2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor.
The capacitive ripple is not in phase with the inductor current. As a result of this, the capacitive ripple does not
decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and
decreases monotonically during off-time. The resistive ripple must exceed the capacitive ripple at the output node
(V_OUT) for stable operation. If this condition is not satisfied, unstable switching behavior is observed in COT
converters, with multiple on-time bursts in close succession followed by a long off-time.
Type 3 ripple method uses R_r and C_r and the switch node (SW) voltage to generate a triangular ramp. This
triangular ramp is ac coupled using C_ac to the feedback node (FB). Since this circuit does not use the output
voltage ripple, it is ideally suited for applications where low output voltage ripple is required. See application note
AN-1481 for more details for each ripple generation method.
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