Datasheet

R
ON
=
V
OUT
x (V
IN
- 1.5V)
F
S
x 1.13 x 10
-10
x V
IN
- 1.4 k:
t
ON
=
V
IN
- 1.5V
1.13 x 10
-10
x (R
ON
+ 1.4 k:)
+ 100 ns
FB
SW
L1
C2
R1
R2
R3
LM34919
BST
VCC
D2
C3
C4
D1
ISEN
SGND
V
OUT
LM34919
www.ti.com SNOSAY2E –MAY 2007–REVISED FEBRUARY 2013
Figure 12. 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 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 13). 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.
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