tON =

VIN - 1.5V

0.565 x 10

-10

x(RON + 1.4 k:)

+ 55 ns

FB

SW

L1

C2

R1

R2

R3

LM34919B

BST

VCC

D2

C3

C4

D1

ISEN

SGND

VOUT

LM34919B

LM34919B-Q1

www.ti.com

SNVS623B – MAY 2010 – REVISED JULY 2013

Startup Regulator, VCC

The start-up regulator is integral to the LM34919B. The input pin (VIN) can be connected directly to line voltage

up to 40V, with transient capability to 44V. The VCC output regulates at 7.0V, and is current limited at 15 mA.

Upon 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 5.25V, the buck switch is enabled and the Softstart pin is

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

The minimum input voltage is determined by the VCC UVLO falling threshold (≊5.1V). When VCC falls below the

falling threshold the VCC UVLO activates to shut off the output. If VCC is externally loaded, the minimum input

voltage increases.

To reduce power dissipation in the startup regulator, an auxiliary voltage can be diode connected to the VCC pin.

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

dissipation. The sum of the auxiliary voltage and the input voltage (VCC + VIN) cannot exceed 52V. Internally, a

diode connects VCC to VIN (see Figure 26).

Figure 26. Self Biased Configuration

Regulation Comparator

The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V). In normal operation (the output

voltage is regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch

stays on for the programmed on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the

buck switch stays off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 100 nA

over temperature.

Over-Voltage Comparator

The voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the on-time

pulse is immediately terminated. This condition can occur if the input voltage or the output load changes

suddenly, or if the inductor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V.

ON-Time Timer, and Shutdown

The on-time is determined by the RON resistor and the input voltage (VIN), and is calculated from:

(5)

The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a specific

continuous conduction mode switching frequency (FS), the RON resistor is determined from the following:

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