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FB

SW

L1

LM5008A

Cff

RFB2

R3

C2

VOUT

RFB1

ON(max)

FB1

FB 2

3 t

Cff =

(R

// R

)

u

16

LM5008A

SNVS583H – MARCH 2009 – REVISED OCTOBER 2018

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C4: The recommended value is 0.01 µF for C4, as this is appropriate in the majority of applications. A high-

quality ceramic capacitor with low ESR is recommended as C4 supplies the surge current to charge the buck

switch gate at turnon. A low ESR also ensures a quick recharge during each off-time. At minimum VIN, when the

on-time is at maximum, it is possible during start-up that C4 does not fully recharge during each 300-ns off-time.

The circuit is not able to complete the start-up, and achieve output regulation. This can occur when the frequency

is intended to be low (for example, RT = 500 K). In this case C4 must be increased so it can maintain sufficient

voltage across the buck switch driver during each on-time.

C5: This capacitor helps avoid supply voltage transients and ringing due to long lead inductance at VIN. A low-

ESR, 0.1-µF ceramic chip capacitor is recommended placed close to the LM5008A.

8.2.2.3 Low-Output Ripple Configurations

For applications where low-output ripple is required, the following options can be used to reduce or nearly

eliminate the ripple:

a. Reduced ripple configuration: In Figure 11, Cff is added across RFB2 to AC-couple the ripple at VOUT

directly to the FB pin. This allows the ripple at VOUT to be reduced to a minimum of 25 mVp-p by reducing

R3, because the ripple at VOUT is not attenuated by the feedback resistors. The minimum value for Cff is

determined by Equation 10:

where

•

tON(max) is the maximum on-time, which occurs at VIN(min). The next larger standard value capacitor must be

used for Cff.

(10)

Figure 11. Reduced Ripple Configuration

b. Minimum ripple configuration: If the application requires a lower value of ripple (<10 mVp-p), the circuit of

Figure 12 can be used. R3 is removed, and the resulting output ripple voltage is determined by the inductor’s

ripple current and C2’s characteristics. RA and CA are chosen to generate a sawtooth waveform at their

junction, and that voltage is AC-coupled to the FB pin through CB. To determine the values for RA, CA, and

CB, use the following procedure in Equation 11:

Calculate VA = VOUT – (VSW × (1 – (VOUT/VIN(min))))

where

•

VSW is the absolute value of the voltage at the SW pin during the off-time (typically 1 V). VA is the DC voltage

at the RA/CA junction, and is used in Equation 12.

(11)

Calculate RA × CA = (VIN(min) – VA) × tON/ΔV

where

•

tON is the maximum on-time (at minimum input voltage), and ΔV is the desired ripple amplitude at the RA/CA

junction (typically 40-50 mV). RA and CA are then chosen from standard value components to satisfy the

above product. Typically CA is 1000 pF to 5000 pF, and RA is 10 kΩ to 300 kΩ. CB is then chosen large

compared to CA, typically 0.1 µF.

(12)