RT8238B

13

RT8238B-00 July 2015

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Copyright 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

maximum average inductor current. Find a low pass

inductor having the lowest possible DC resistance that

fits in the allowed dimensions. Ferrite cores are often the

best choice, although powdered iron is inexpensive and

can work well at 200kHz. The core must be large enough

IR

PEAK

LOAD(MAX)

LOAD(MAX)

L

I

= I

+

I

2



 









Output Capacitor Selection

The output filter capacitor must have low enough Equivalent

Series Resistance (ESR) to meet output ripple and load-

transient requirements, yet have high enough ESR to

satisfy stability requirements. The output capacitance

must also be high enough to absorb the inductor energy

while transiting from full-load to no-load conditions without

tripping the overvoltage fault latch.

provides many advantages such as ease-of-use, minimum

external component configuration, and extremely short

response time, due to not employing an error amplifier in

the loop, a sufficient feedback signal needs to be provided

by an external circuit to reduce the jitter level. The required

SW

ESR

OUT

f

1

f

=

2 π ESR C

4







Do not put high value ceramic capacitors directly across

the outputs without taking precautions to ensure stability.

Large ceramic capacitors can have a high-ESR zero

frequency and cause erratic and unstable operation.

However, it is easy to add sufficient series resistance by

placing the capacitors a couple of inches downstream from

the inductor and connecting VOUT or FB divider close to

the inductor. There are two related but distinct ways

including double pulsing and feedback loop instability to

identify the unstable operation. Double pulsing occurs due

to noise on the output or because the ESR is too low that

there is not enough voltage ramp in the output voltage

signal. This “fools” the error comparator into triggering a

new cycle immediately after a 400ns minimum off-time

period has expired. Double pulsing is more annoying than

harmful, resulting in nothing worse than increased output

ripple. However, it may indicate the possible presence of

loop instability, which is caused by insufficient ESR. Loop

instability can result in oscillation at the output after line

or load perturbations that can trip the over voltage

protection latch or cause the output voltage to fall below

the tolerance limit. The easiest method for stability

checking is to apply a very zero-to-max load transient

and carefully observe the output voltage ripple envelope

for overshoot and ringing. It helps to simultaneously monitor

the inductor current with AC probe. Do not allow more

than one ringing cycle after the initial step-response under-

or over shoot.

OUT

REF

R1

V

= V

1+

R2



 



Figure 4. Setting VOUT with a Resistor-Divider

Output Inductor Selection

The switching frequency (on-time) and operating point (%

ripple or LIR) determine the inductor value as follows :





ON

IN

OUT

IR

LOAD(MAX)

TV

V

L =

LI





R2 to be approximately 10k

Ω, and solve for R1 using the

equation :

signal level is approximately 15mV at the comparing point.

node. The output capacitor ESR should meet this

requirement.

Output Capacitor Stability

Stability is determined by the value of the ESR zero relative

to the switching frequency. The point of instability is given

by the following equation :

Thermal Considerations

For continuous operation, do not exceed absolute

maximum operation junction temperature. The maximum

power dissipation depends on the thermal resistance of

PHASE

LGATE

R1

R2

UGATE

FB

GND