REF

FB2

FB1

OUT

FB1

V

(R

R

)

V

V

R

u

OUT

sw

10

ON

V

F

Hz

R

1 10

u u

12

LM5160A, LM5160

SNVSA03C – OCTOBER 2014 – REVISED AUGUST 2016

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Copyright © 2014–2016, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 Control Circuit

The LM5160 step-down switching regulator employs a control principle based on a comparator and a one-shot

minimum OFF-time forced by the minimum OFF-time one-shot. The buck switch remains off until the FB voltage

falls below the soft start again, when it turns back on for another ON-time one-shot period.

During a rapid start-up or when the load current increases suddenly, the regulator operates with minimum OFF-

time per cycle. When regulating the output in steady-state operation, the OFF-time automatically adjusts to

produce the SW pin duty cycle required for output voltage regulation.

When in regulation, the LM5160 operates in continuous conduction mode at heavy load currents. If the FPWM

pin is connected to ground or left floating, the regulator operates in discontinuous conduction mode at light load

with the synchronous rectifier FET in diode emulation. With sufficient load, the LM5160 operates in continuous

conduction mode with the inductor current never reaching zero during the OFF-time of the high-side FET. In this

mode the operating frequency remains relatively constant with load and line variations. The minimum load

current for continuous conduction mode is one-half the inductor’s ripple current amplitude. The operating

ohms.

(1)

In discontinuous conduction mode, current through the inductor ramps up from zero to a peak value during the

ON-time, then ramps back to zero before the end of the OFF-time. The next ON-time period starts when the

current is supplied by the output capacitor. In this mode, the operating switching frequency is lower than the

continuous conduction mode switching frequency and the frequency varies with load. The discontinuous

conduction mode maintains conversion efficiency at light loads because the switching losses decrease with the

decrease in load and frequency.

Equation 2.

where

•

(2)

7.3.2 VCC Regulator

The LM5160 contains an internal high voltage linear regulator with a nominal output voltage of 7.5 V (typical).

The VCC regulator is internally current limited to 30 mA (minimum). This regulator supplies power to internal

circuit blocks including the synchronous FET gate driver and the logic circuits. When the voltage on the VCC pin

at the VCC pin stabilizes the regulator and supplies transient VCC current to the gate drivers. An internal diode

connected from VCC to the BST pin replenishes the charge in the high-side gate drive bootstrap capacitor when

the SW pin is low.

In high input voltage applications, the power dissipated in the regulator is significant and can limit the efficiency

and maximum achievable output power. The LM5160A allows the internal VCC regulator power loss to be

reduced by supplying the VCC voltage through a diode from an external voltage source regulated between 9 V

and 13 V. The external VCC bias can be supplied from the LM5160A converter output rail if the regulation

voltage is within this range. When the VCC pin of the LM5160A is raised above the regulation voltage (7.5 V

typical), the internal regulator is disabled and the power dissipation in the IC is reduced. The only difference