LM4510 Datasheet, PDF (16/30 Pages) National Semiconductor (TI) – Synchronous Step-Up DC/DC Converter with True Shutdown Isolation

LM4510
SNVS533D – SEPTEMBER 2007 – REVISED NOVEMBER 2014
www.ti.com
Table 3. Suggested C_IN and C_OUT Capacitors and Their Suppliers
MODEL
TYPE
VENDOR
VOLTAGE
RATING
4.7 µF for C_IN
C2012X5R0J475
Ceramic, X5R
TDK
6.3 V
GRM21BR60J475
Ceramic, X5R
muRata
6.3 V
JMK212BJ475
Ceramic, X5R
Taiyo-Yuden
6.3 V
C2012X5R0J475K
Ceramic, X5R
TDK
6.3 V
10 µF for C_OUT
TMK316BJ106KL
Ceramic, X5R
Taiyo-Yuden
25 V
12103D106KAT2A
Ceramic, X5R
AVX
25 V
CASE SIZE
INCH (mm)
0805 (2012)
0805 (2012)
0805 (2012)
0603 (1608)
1206 (3216)
1210 (3225)
8.2.1.2.7 Soft-Start Function and Soft-Start Capacitor Selection
The LM4510 has a soft-start pin that can be used to limit the input inrush current. Connect a capacitor from SS
pin to GND to set the soft-start period. Figure 19 describes the soft start process.
• Initial charging period: When the device is turned on, the control circuitry linearly regulating initial charge
current charges V_OUT by limiting the inrush current.
• Soft-start period: After V_OUT reaches V_IN –0.7 V (typ.), the device starts switching and the C_S is charged at a
constant current of 11 μA, ramping up to V_IN. This period ends when V_SS reaches V_FB. C_S should be large
enough to ensure soft-start period ends after C_O is fully charged.
During the initial charging period, the required load current must be smaller than the initial charge current to
ensure V_OUT reaches V_IN –0.7 V (typ.).
V_EN
V_SS
V_SS = V_IN
V_SS = V_FB
appropriate V_OUT
V_OUT
V_OUT = V_IN - 0.7V
Figure 19. Soft-Start Timing Diagram
C_S Selection
The soft-start time without load can be estimated as:
t_SS
=
C_OUT x (V_IN - 0.7)
I_{INIT_ CHARGE}
+
C_S x V_FB
I_{SS _ CHARGE}
[sec]
(8)
Where the I_{INIT_CHARGE} is Initial Charging Current depending on V_IN and I_{SS_CHARGE} (11 μA (typ.). Also, when
selecting the fuse current rating, make sure the value is higher than the initial charging current.
16
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