LM34919_14 Datasheet, PDF (9/21 Pages) Texas Instruments – Ultra-Small 40-V 600-mA Constant On-Time Buck Switching Regulator

www.ti.com
LM34919
SNOSAY2D – MAY 2007 – REVISED JUNE 2008
LM34919
VCC
C3
BST
C4
L1
SW
D1
ISEN
SGND
FB
D2
V_OUT
R1
R3
R2
C2
Figure 4. 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 R_ON resistor and the input voltage (V_IN), and is calculated from:
1.13 x 10^-10 x (R_ON + 1.4 k:)
t_ON =
+ 100 ns
V_IN - 1.5V
(5)
The inverse relationship with V_IN results in a nearly constant frequency as V_IN is varied. To set a specific
continuous conduction mode switching frequency (F_S), the R_ON resistor is determined from the following:
R_ON
=
V_OUT x (V_IN - 1.5V)
F_S x 1.13 x 10^-10 x V_IN
-
1.4
k:
(6)
In high frequency applications the minimum value for t_ON is limited by the maximum duty cycle required for
regulation and the minimum off-time of (155 ns, ±15%). The minimum off-time limits the maximum duty cycle
achievable with a low voltage at V_IN. At high values of VIN, the minimum on-time is limited to ≊ 120 ns.
The LM34919 can be remotely shut down by taking the RON/SD pin below 0.8V (see Figure 5). In this mode the
SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the RON/SD pin
allows normal operation to resume. The voltage at the RON/SD pin is between 1.4V and 4.0V, depending on V_IN
and the R_ON resistor.
Copyright © 2007–2008, Texas Instruments Incorporated
Product Folder Links: LM34919
Submit Documentation Feedback
9