tSS =

C6 x 2.5V

11.5 PA

C1 =

IO x tON

'V

ESR(min) =

25 mV x (R1 + R2)

R2 x IOR(min)

LM34910C

www.ti.com

SNVS517B – MAY 2007 – REVISED MARCH 2013

C2, and R3: Since the LM34910C 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 C2’s 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:

(14)

If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Typical Application

Circuit and Block Diagram. Generally R3 is less than 1

Ω. C2 should generally be no smaller than 3.3 µF,

although that is dependent on the frequency and the allowable ripple amplitude at VOUT1. Experimentation is

usually necessary to determine the minimum value for C2, as the nature of the load may require a larger value. A

load which creates significant transients requires a larger value for C2 than a non-varying load.

D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time

determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage

drop is significant in the event the output is short-circuited as it is mainly this diode’s voltage (plus the voltage

across the current limit sense resistor) which forces the inductor current to decrease during the off-time. For this

reason, a higher voltage is better, although that affects efficiency. A reverse recovery time of

≊30 ns, and a

forward voltage drop of

≊0.75V are preferred. The reverse leakage specification is important as that can

significantly affect efficiency. D1’s reverse voltage rating must be at least as great as the maximum VIN, and its

current rating must equal or exceed IPK Figure 6.

C1 and C5: C1’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple

at VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero. If the

source’s dynamic impedance is high (effectively a current source), it supplies the average input current, but not

the ripple current.

At maximum load current, when the buck switch turns on, the current into VIN suddenly increases to the lower

peak of the inductor’s ripple current, ramps up to the peak value, then drop to zero at turn-off. The average

current during the on-time is the load current. For a worst case calculation, C1 must supply this average load

current during the maximum on-time. C1 is calculated from:

where

•

Io is the load current

•

tON is the maximum on-time

•

ΔV is the allowable ripple voltage at VIN

(15)

C5’s purpose is to help avoid transients and ringing due to long lead inductance at VIN. A low ESR, 0.1 µF

ceramic chip capacitor is recommended, located close to the LM34910C .

C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended

as C4 supplies a surge current to charge the buck switch gate at turn-on. A low ESR also helps ensure a

complete recharge during each off-time.

C6: The capacitor at the SS pin determines the softstart time, i.e. the time for the reference voltage at the

regulation comparator, and the output voltage, to reach their final value. The time is determined from the

following:

(16)

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