2

x L1 x 1.57 x 10

20

2

DC =

=

1.13 x 10

-10

LM34919

SNOSAY2D – MAY 2007 – REVISED JUNE 2008

www.ti.com

Control Circuit Overview

The LM34919 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

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 LM34919 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)

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

follows:

(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 LM34919 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).

The start-up regulator is integral to the LM34919. The input pin (VIN) can be connected directly to line voltage up

power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC

pin reaches the under-voltage lockout threshold of 5.7V, the buck switch is enabled and the Softstart pin is

released to allow the Softstart capacitor (C6) to charge up.

(

Setting the auxiliary voltage to between 7V and 14V shuts off the internal regulator, reducing internal power

diode connects VCC to VIN (see Figure 4).

8

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