LM34910_15 Datasheet, PDF(10/18 Page) Texas Instruments – High Voltage (40V, 1.25A) Step Down Switching Regulator

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LM34910_15 Datasheet, PDF (10/18 Pages) Texas Instruments – High Voltage (40V, 1.25A) Step Down Switching Regulator

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LM34910

SNVS297B â OCTOBER 2004 â REVISED MARCH 2013

APPLICATIONS INFORMATION

www.ti.com

EXTERNAL COMPONENTS

The following guidelines can be used to select the external components.

R1 and R2: The ratio of these resistors is calculated from:

R1/R2 = (VOUT/2.5V) - 1

(8)

R1 and R2 should be chosen from standard value resistors in the range of 1.0 kâ¦ - 10 kâ¦ which satisfy the

above ratio.

RON: The minimum value for RON is calculated from:

200 ns x VINMAX

t RON

1.3 x 10-10

(9)

Equation 1 can be used to select RON if a specific frequency is desired as long as the above limitation is met.

L1: The main parameter affected by the inductor is the output current ripple amplitude (IOR). The limits for IOR

must be determined at both the minimum and maximum nominal load currents.

a) If the maximum load current is less than the current limit threshold (1.25A), the minimum load current is used

to determine the maximum allowable ripple. To maintain continuous conduction mode the lower peak should not

reach 0 mA. For this case, the maximum ripple current is:

IOR(MAX1) = 2 x IO(min)

(10)

The ripple calculated in Equation 10 is then used in the following equation:

VOUT x (VIN - VOUT)

L1 =

IOR x FS x VIN

(11)

where VIN is the maximum input voltage and Fs is determined from Equation 1. This provides a minimum value

for L1. The next larger standard value should be used, and L1 should be rated for the IPK current level.

b) If the maximum load current is greater than the current limit threshold (1.25A), the LM34910 ensures the lower

peak reaches 1.25A each cycle, requiring that IOR be at least twice the difference. The upper peak, however,

must not exceed 3.5A. For this case, the ripple limits are:

IOR(MAX2) = 2 x (3.5A - IO(max))

(12)

and

IOR(MIN1) = 2 x (IO(max) - 1.25A)

(13)

The lesser of Equation 12 and Equation 13 is then used in Equation 11. If IOR(MAX2) is used, the maximum VIN is

used in Equation 11. The next larger value should then be used for L1. If IOR(MIN1) is used, the minimum VIN is

used in Equation 11. The next smaller value should then be used for L1. L1 must be rated for the peak value of

the current waveform (IPK in Figure 7).

C3: The capacitor on the VCC output provides not only noise filtering and stability, but also prevents false

triggering of the VCC UVLO at the buck switch on/off transitions. For this reason, C3 should be no smaller than

0.1 ÂµF, and should be a good quality, low ESR, ceramic capacitor.

C2, and R3: Since the LM34910 requires a minimum of 25 mVp-p of ripple at the FB pin for proper operation, the

required ripple at VOUT1 is increased by R1 and R2. This necessary ripple is created by the inductor ripple current

acting on C2âs ESR + R3. The minimum ripple current is calculated using Equation 11, rearranged to solve for

IOR at minimum VIN. The minimum ESR for C2 is then equal to:

25 mV x (R1 + R2)

ESR(min) =

R2 x IOR(min)

(14)

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