Datasheet
R
ON
=
V
OUT
x (V
IN
- 1.5V)
F
S
x 0.565 x 10
-10
x V
IN
- 1.4 k:
t
ON
=
V
IN
- 1.5V
0.565 x 10
-10
x(R
ON
+ 1.4 k:)
+ 55 ns
FB
SW
L1
C2
R1
R2
R3
LM34919C
BST
VCC
D2
C3
C4
D1
ISEN
SGND
V
OUT
LM34919B
LM34919B-Q1
www.ti.com
SNVS623A –MAY 2010–REVISED FEBRUARY 2013
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 R
ON
resistor and the input voltage (V
IN
), and is calculated from:
(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:
(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. 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 ≊ 90 ns.
The LM34919B can be remotely shut down by taking the RON/SD pin low (see Figure 27). 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 5.0V, depending on V
IN
and the R
ON
resistor.
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