FS =

VOUT

2

x L1 x 6.27 x 10

20

RL x (RON)

2

DC =

tON

tON + tOFF

=

VOUT

VIN

FS =

VOUT x (VIN ± 1.5V)

0.565 x 10

-10

x (RON + 1.4 k:) x VIN

LM34919B

LM34919B-Q1

SNVS623B – MAY 2010 – REVISED JULY 2013

www.ti.com

FUNCTIONAL DESCRIPTION

The LM34919B Step Down Switching Regulator features all the functions needed to implement a low-cost,

efficient buck bias power converter capable of supplying at least 0.6A to the load. This high voltage regulator

contains an N-Channel buck switch, is easy to implement, and is available in a DSBGA package. The regulator’s

operation is based on a constant on-time control scheme, where the on-time is determined by VIN. This feature

allows the operating frequency to remain relatively constant with load and input voltage variations. The feedback

control requires no loop compensation resulting in very fast load transient response. The valley current limit

detection circuit, internally set at 0.64A, holds the buck switch off until the high current level subsides. This

scheme protects against excessively high current if the output is short-circuited when VIN is high.

The LM34919B can be applied in numerous applications to efficiently regulate down higher voltages. Additional

features include: Thermal shutdown, VCC under-voltage lockout, gate drive under-voltage lockout, and maximum

duty cycle limiter.

Control Circuit Overview

The LM34919B buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-

timer, with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB voltage is below

the reference the buck switch is turned on for a time period determined by the input voltage and a programming

resistor (RON). Following the on-time the switch remains off until the FB voltage falls below the reference but not

less than the minimum off-time. The buck switch then turns on for another on-time period. Typically, during start-

up, or when the load current increases suddenly, the off-times are at the minimum. Once regulation is

established, the off-times are longer.

When in regulation, the LM34919B operates in continuous conduction mode at heavy load currents and

discontinuous conduction mode at light load currents. In continuous conduction mode current always flows

through the inductor, never reaching zero during the off-time. 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 frequency is approximately:

(1)

The buck switch duty cycle is approximately equal to:

(2)

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

then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB

falls below the reference - until then the inductor current remains zero, and the load current is supplied by the

output capacitor. In this mode the operating frequency is lower than in continuous conduction mode, and varies

with load current. Conversion efficiency is maintained at light loads since the switching losses decrease with the

reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as

follows:

(3)

where RL = the load resistance.

The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as

follows:

VOUT = 2.5 x (R1 + R2) / R2

(4)

Output voltage regulation is based on ripple voltage at the feedback input, normally obtained from the output

voltage ripple through the feedback resistors. The LM34919B requires a minimum of 25 mV of ripple voltage at

the FB pin. In cases where the capacitor’s ESR is insufficient additional series resistance may be required (R3).

12

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