FB

SW

L1

C2

R1

R2

R3

LM34910C

BST

VCC

D2

C3

C4

D1

ISEN

SGND

VOUT1

VOUT2

FB

SW

L1

C2

R1

R2

VOUT2

R3

LM34910C

LM34910C

www.ti.com

SNVS517B – MAY 2007 – REVISED MARCH 2013

Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR

for the output capacitor C2. The LM34910C 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 3. However, R3 slightly degrades the load regulation.

Figure 3. Low Ripple Output Configuration

Start-up Regulator, VCC

The start-up regulator is integral to the LM34910C. The input pin (VIN) can be connected directly to line voltage

up to 50V, with transient capability to 55V. The VCC output regulates at 7.0V, and is current limited to 9 mA. Upon

power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC 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 VCC UVLO falling threshold

(

≊5.7V), and the frequency. When VCC falls below the falling threshold the VCC UVLO activates to shut off the

output. If VCC is externally loaded, the minimum input voltage increases since the output impedance at VCC is

≊140Ω. See Figure 1.

To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the VCC 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 (VCC + VIN) cannot exceed 70V. Internally, a

diode connects VCC to VIN. See Figure 4.

Figure 4. Self Biased Configuration

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