(2)

Where I

Example: V

(3)

R

(4)

The output voltage is limited by the maximum duty cycle as

well as the minimum on time. Figure 5 shows the limits for

input and output voltages. The recommended maximum out-

put voltage is approximately 1V less than the nominal input

voltage. At 30V input, the minimum output is approximately

2.3V and the maximum is approximately 27V. For input

voltages below 5.5V, VLIN5 must be connected to VIN

through a small resistor (approximately 4.7

Ω). Doing this

ensures that VLIN5 does not fall below the UVLO threshold.

Output Capacitor Selection

In applications that exhibit large and fast load current

swings, the slew rate of such a load current transient may be

beyond the response speed of the regulator. Therefore, to

meet voltage transient requirements during worst-case load

transients, special consideration should be given to output

capacitor selection. The total combined ESR of the output

capacitors must be lower than a certain value, while the total

capacitance must be greater than a certain value. Also, in

applications where the specification of output voltage regu-

lation is tight and ripple voltage must be low, starting from the

required output voltage ripple (

∆V

design iterations.

ALLOWED TRANSIENT VOLTAGE EXCURSION

The allowed output voltage excursion during a load transient

(

∆V

(5)

Where

δ% is the output voltage regulation window, e%isthe

output voltage initial accuracy V

voltage, and

∆V

Example: V

δ% = 7%, e% = 3.4%, ∆V

peak-to-peak.

(6)

Since the ripple voltage is included in the calculation of

∆V

in the worst-case load current excursion. That is, the worst-

case load current excursion should be simply maximum load

current change specification,

∆I

MAXIMUM ESR CALCULATION

Unless the rise and fall times of a load transient are slower

than the response speed of the control loop, if the total

combined ESR (R

ment is not met, no matter how large the capacitance. The

maximum allowed total combined ESR is:

(7)

Example:

∆V

∆I

53.3m

Ω.

Maximum ESR criterion can be used when the associated

capacitance is high enough, otherwise more capacitors than

the number determined by this criterion should be used in

parallel.

MINIMUM CAPACITANCE CALCULATION

In a switch mode power supply, the minimum output capaci-

tance is typically dictated by the load transient requirement.

If there is not enough capacitance, the output voltage excur-

sion will exceed the maximum allowed value even if the

maximum ESR requirement is met. The worst-case load

transient is an unloading transient that happens when the

20162312

FIGURE 4. Output Voltage Setting

20162314

FIGURE 5. Available Output Voltage Range

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