English
Language : 

LM34927SD Datasheet, PDF (14/26 Pages) Texas Instruments – LM34927 Integrated Secondary Side Bias Regulator for Isolated DC-DC Converters
◁
LM34927
SNVS799F – APRIL 2012 – REVISED DECEMBER 2013
Type 1
Lowest Cost Configuration
Table 3. Ripple Configuration
Type 2
Reduced Ripple Configuration
L1
RFB2
VOUT
RC
VOUT
L1
Cac
RFB2 RC
To FB
R FB1
C OUT
To FB
R FB1
C OUT
GND
RC >
25 mV
ûIL(MIN)
x
VOUT
VREF
(7)
GND
C
>
5
Csw(RFB2||RFB1)
RC
>
25 mV
ûIL(MIN)
(8)
www.ti.com
Type 3
Minimum Ripple Configuration
VOUT
L1
Rr
Cac
To FB
Cr RFB2
R FB1
C OUT
GND
Cr = 3300 pF
Cac = 100 nF
RrCr <
(VIN(MIN) - VOUT) x TON
25 mV
(9)
Soft Start
A soft-start feature can be implemented to the LM34927 using an external circuit. As shown in Figure 19, the
soft-start circuit consists of one capacitor, C1, two resistors, R1 and R2, and a diode, D. During the initial start-up,
the VCC voltage is established prior to the VOUT voltage. D is thereby forward biased and the FB voltage is pulled
up above the reference voltage (1.225V). The switcher is disabled. With the charging of the capacitor C1, the
voltage at node B gradually decreases. Due to the action of the control circuit, VOUT will gradually rise to maintain
the FB voltage at the reference voltage. Once the voltage at node B is lower than the FB voltage, plus the
voltage drop of D, the soft-start is finished and D is reverse biased.
During the initial part of the start-up, the FB voltage can be approximated as follows. Please note that the effect
of R1 has been ignored to simplify the calculation:
VFB
=
(VCC
-
VD)
x
R2
x
(RFB1
RFB1 x RFB2
+ RFB2) + RFB1
x
RFB2
(10)
To achieve the desired soft-start, the following design guidance is recommended:
1. R2 is selected so that VFB is higher than 1.225V for a VCC of 4.5V, but is lower than 5V when VCC is 8.55V. If
an external VCC is used, VFB should not exceed 5V at maximum VCC.
2. C1 is selected to achieve the desired start-up time that can be determined as:
tS
=
C1
x
(R2
+
RFB1
RFB1
x
+
RFB2
RFB2
)
(11)
3. R1 is used to maintain the node B voltage at zero after the soft-start is finished. A value larger than the
feedback resistor divider is preferred.
Based on the schematic shown in Figure 17, selecting C1 = 1 uF, R2=1 kΩ, R1=30 kΩ results in a soft-start
time of about 2ms.
14
Submit Documentation Feedback
Product Folder Links: LM34927
Copyright © 2012–2013, Texas Instruments Incorporated
 ▷