Datasheet

STOP
RUN
R
ON
/SD
Input
Voltage
LM34910C
V
IN
R
ON
F
MAX
=
V
OUT
V
INMAX
x 200 ns
t
ON
=
V
IN
1.3 x 10
-10
x R
ON
LM34910C
SNVS517B –MAY 2007–REVISED MARCH 2013 www.ti.com
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 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. Bias current at the FB pin is nominally 100 nA.
Over-Voltage Comparator
The voltage at FB is compared to an internal 2.875V reference. If the voltage at FB rises above 2.875V 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 for the LM34910C is determined by the R
ON
resistor and the input voltage (V
IN
), and is calculated
from:
(5)
See Figure 2. The inverse relationship with V
IN
results in a nearly constant frequency as V
IN
is varied. R
ON
should
be selected for a minimum on-time (at maximum V
IN
) greater than 200 ns. This requirement limits the maximum
frequency for each application, depending on V
IN
and V
OUT
, calculated from the following:
(6)
The LM34910C can be remotely shut down by taking the R
ON
/SD pin below 0.65V. See Figure 5. In this mode
the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the R
ON
/SD
pin allows normal operation to resume. The voltage at the R
ON
/SD pin is between 1.5V and 3.0V, depending on
V
IN
and the R
ON
resistor.
Figure 5. Shutdown Implementation
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling
diode (D1). Referring to Typical Application Circuit and Block Diagram, when the buck switch is turned off the
inductor current flows through the load, into S
GND
, through the sense resistor, out of I
SEN
and through D1. If that
current exceeds 1.25A the current limit comparator output switches to delay the start of the next on-time period if
the voltage at FB is below 2.5V. The next on-time starts when the current out of I
SEN
is below 1.25A and the
voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed 1.25A
during each on-time, that is detected at the beginning of each off-time. The operating frequency may be lower
due to longer-than-normal off-times.
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