LM34910_15 Datasheet, PDF (7/18 Pages) Texas Instruments – High Voltage (40V, 1.25A) Step Down Switching Regulator

LM34910
www.ti.com
SNVS297B – OCTOBER 2004 – REVISED MARCH 2013
The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as
follows:
V_OUT = 2.5 x (R1 + R2) / R2
(4)
Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR
for the output capacitor C2. The LM34910 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases
where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Typical Application
Circuit and Block Diagram).
For applications where lower output voltage ripple is required the output can be taken directly from a low ESR
output capacitor as shown in Figure 4. However, R3 slightly degrades the load regulation.
L1
SW
LM34910
FB
R1
R3
V_OUT2
R2
C2
Figure 4. Low Ripple Output Configuration
Start-up Regulator, V_CC
The start-up regulator is integral to the LM34910. The input pin (V_IN) can be connected directly to line voltage up
to 36V, with transient capability to 40V. The V_CC output regulates at 7.0V, and is current limited to 9 mA. Upon
power up, the regulator sources current into the external capacitor at V_CC (C3). When the voltage on the V_CC pin
reaches the under-voltage lockout threshold of 5.8V, the buck switch is enabled and the Softstart pin is released
to allow the Softstart capacitor (C6) to charge up.
The minimum input voltage is determined by the regulator’s dropout voltage, the V_CC UVLO falling threshold
(≊5.7V), and the frequency. When V_CC falls below the falling threshold the V_CC UVLO activates to shut off the
output. If V_CC is externally loaded, the minimum input voltage increases since the output impedance at V_CC is
≊140Ω. See Figure 2.
To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the V_CC pin.
Setting the auxiliary voltage to between 8V and 14V shuts off the internal regulator, reducing internal power
dissipation. The sum of the auxiliary voltage and the input voltage (V_CC + V_IN) cannot exceed 50V. Internally, a
diode connects V_CC to V_IN. See Figure 5.
V_CC
C3
BST
LM34910
C4
L1
SW
D1
I_SEN
S_GND
FB
D2
V_OUT1
R1
R3
V_OUT2
R2
C2
Figure 5. Self Biased Configuration
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