VIN

VIN

0.1 2F

AGND

1

6

L

COUT

SW

VCCX

VOUT

CVCCX

LM5116-HT

www.ti.com

SLVSBS8B – OCTOBER 2013 – REVISED JANUARY 2014

Upon power-up, the regulator sources current into the capacitor connected to the VCC pin. When the voltage at

the VCC pin exceeds 4.5 V and the UVLO pin is greater than 1.215 V, the output switch is enabled and a soft-

start sequence begins. The output switch remains enabled until VCC falls below 4.5 V, EN is pulled low, the

UVLO pin falls below 1.215 V or the die temperature exceeds the thermal limit threshold.

Figure 3. VCCX Bias Supply with Additional Inductor Winding

An output voltage derived bias supply can be applied to the VCCX pin to reduce the IC power dissipation. If the

bias supply voltage is greater than 4.5 V, the internal regulator will essentially shut off, reducing the IC power

dissipation. The VCC regulator series pass transistor includes a diode between VCC and VIN that should not be

forward biased in normal operation. For an output voltage between 5 V and 15 V, VOUT can be connected

directly to VCCX. For VOUT < 5 V, a bias winding on the output inductor can be added to VOUT. If the bias

winding can supply VCCX greater than VIN, an external blocking diode is required from the input power supply to

the VIN pin to prevent VCC from discharging into the input supply.

The output of the VCC regulator is current limited to 15 mA minimum. The VCC current is determined by the

MOSFET gate charge, switching frequency and quiescent current (see MOSFETs section in the Application

Information). If VCCX is powered by the output voltage or an inductor winding, the VCC current should be

evaluated during startup to ensure that it is less than the 15 mA minimum current limit specification. IF VCCX is

powered by an external regulator derived from VIN, there is no restriction on the VCC current.

Figure 4. Input Blocking Diode for VCCX > VIN

In high voltage applications extra care should be taken to ensure the VIN pin does not exceed the absolute

maximum voltage rating of 80 V. During line or load transients, voltage ringing on the VIN line that exceeds the

Absolute Maximum Ratings can damage the IC. Both careful PC board layout and the use of quality bypass

capacitors located close to the VIN and GND pins are essential.

Enable

The LM5116 contains an enable function allowing a very low input current shutdown. If the enable pin is pulled

below 0.5 V, the regulator enters shutdown, drawing less than 10 µA from the VIN pin. Raising the EN input

above 3.3 V returns the regulator to normal operation. The maximum EN transition time for proper operation is

one switching period. For example, the enable rise time must be less than 4

μs for 250 kHz operation.

A 1 M

Ω pull-up resistor to VIN can be used to interface with an open collector control signal. At low input voltage

the pull-up resistor may be reduced to 100 k

Ω to speed up the EN transition time. The EN pin can be tied directly

to VIN if this function is not needed. It must not be left floating. If low-power shutdown is not needed, the UVLO

pin should be used as an on/off control.

Copyright © 2013–2014, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM5116-HT