Datasheet

VCC
LM25117
V
OUT
Zener
R
1
R1 is required to limit maximum reverse zener current
30 k: minimum resistive loss at VCC
guarantees minimum reverse zener current
30 k:
VCC
VCCDIS
LM25117
V
OUT
VCCDIS resistor divider is required
when external VCC supplying voltage is
smaller than 8.5V
SW
VCC
VCCDIS
LM25117
V
OUT
VCCDIS resistor divider is required
when external VCC supplying voltage is
smaller than 8.5V
LM25117
LM25117-Q1
SNVS714E –APRIL 2011–REVISED MARCH 2013 www.ti.com
Figure 19. External VCC Supply for 5V< V
OUT
<14.5V
For V
OUT
< 5V, a bias winding on the output inductor can be added to generate the external VCC supply voltage.
Figure 20. External VCC Supply for V
OUT
<5V
For 14.5V <V
OUT
, the external supply voltage can be regulated by using a series Zener diode from the output to
VCC.
Figure 21. External VCC Supply for 14.5V <V
OUT
In high input voltage applications, extra care should be taken to ensure the VIN pin does not exceed the absolute
maximum voltage rating of 45V. During line or load transients, voltage ringing on the VIN that exceeds the
Absolute Maximum Rating can damage the IC. Both careful PC board layout and the use of quality bypass
capacitors located close to the VIN and AGND pin are essential. Adding an R-C filter (R
VIN
, C
VIN
) on VIN is
optional and helps to prevent faulty operation caused by poor PC board layout and high frequency switching
noise injection. The recommended capacitance and resistance range are 0.1µF to 10µF and 1Ω to 10Ω.
UVLO
The LM25117 contains a dual level UVLO (under-voltage lockout) circuit. When the UVLO is less than 0.4V, the
LM25117 is in shutdown mode. The shutdown comparator provides 100mV of hysteresis to avoid chatter during
transitions. When the UVLO pin voltage is greater than 0.4V but less than 1.25V, the controller is in standby
mode. In the standby mode, the VCC bias regulator is active but the HO and LO drivers are disabled and the SS
pin is held low. This feature allows the UVLO pin to be used as a remote shutdown function by pulling the UVLO
pin down below 0.4V with an external open collector or open drain device. When the VCC pin exceeds its under-
voltage lockout threshold and the UVLO pin voltage is greater than 1.25V, the HO and LO drivers are enabled
and normal operation begins.
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