LM34910_15 Datasheet, PDF (8/18 Pages) Texas Instruments – High Voltage (40V, 1.25A) Step Down Switching Regulator

LM34910
SNVS297B – OCTOBER 2004 – REVISED MARCH 2013
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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 LM34910 is determined by the R_ON resistor and the input voltage (V_IN), and is calculated
from:
1.3 x 10^-10 x R_ON
t_ON =
V_IN
(5)
See Figure 3. 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:
V_OUT
F_MAX = _{VINMAX x 200 ns}
(6)
The LM34910 can be remotely shut down by taking the R_ON/SD pin below 0.65V. See Figure 6. 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.
Input
Voltage
V_IN
R_ON
R_ON/SD
STOP
RUN
LM34910
Figure 6. 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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