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KB3511

Kingbor Technology Co.,Ltd

TEL:(86)0755-26508846 FAX:(86)0755-26509052

APPLICATIONS INFORMATION

capacitors, such as Sanyo POSCAP, offer very low ESR,

but have a lower capacitance density than other types.

Tantalum capacitors have the highest capacitance density,

but it has a larger ESR and it is critical that the capacitors

are surge tested for use in switching power supplies. An

excellent choice is the AVX TPS series of surface mount

tantalums, available in case heights ranging from 2mm to

4mm. Aluminum electrolytic capacitors have a signifi-

cantly larger ESR, and are often used in extremely cost-

sensitive applications provided that consideration is given

to ripple current ratings and long term reliability. Ceramic

capacitors have the lowest ESR and cost, but also have the

lowest capacitance density, a high voltage and tempera-

ture coefficient, and exhibit audible piezoelectric effects.

In addition, the high Q of ceramic capacitors along with

trace inductance can lead to significant ringing. Other

capacitor types include the Panasonic Special Polymer

(SP) capacitors.

In most cases, 0.1

µF to 1µF of ceramic capacitors should

also be placed close to the KB3511 in parallel with the

main capacitors for high frequency decoupling.

Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now

becoming available in smaller case sizes. These are tempt-

ing for switching regulator use because of their very low

ESR. Unfortunately, the ESR is so low that it can cause

loop stability problems. Solid tantalum capacitor ESR

generates a loop “zero” at 5kHz to 50kHz that is instrumen-

tal in giving acceptable loop phase margin. Ceramic ca-

pacitors remain capacitive to beyond 300kHz and usually

resonate with their ESL before ESR becomes effective.

Figure 2. kB3511 General Schematic

RUN2

TO 5.5V

RUN1

POR

SW2

GND

SW1

MODE/SYNC

KB3511ADJ

R5

POWER-ON

RESET

C4

C5

L1

L2

R4

R2

R1

R3

PWM*

Trickle*

*MODE/SYNC = 0V: PWM

Also, ceramic caps are prone to temperature effects which

requires the designer to check loop stability over the

operating temperature range. To minimize their large

temperature and voltage coefficients, only X5R or X7R

ceramic capacitors should be used. A good selection of

ceramic capacitors is available from Taiyo Yuden, TDK,

and Murata.

Great care must be taken when using only ceramic input

and output capacitors. When a ceramic capacitor is used

at the input and the power is being supplied through long

wires, such as from a wall adapter, a load step at the output

couple to the output and be mistaken as loop instability. At

worst, the ringing at the input can be large enough to

damage the part.

Since the ESR of a ceramic capacitor is so low, the input

and output capacitor must instead fulfill a charge storage

requirement. During a load step, the output capacitor must

instantaneously supply the current to support the load

until the feedback loop raises the switch current enough to

support the load. The time required for the feedback loop

to respond is dependent on the compensation and the

output capacitor size. Typically, 3-4 cycles are required to

respond to a load step, but only in the first cycle does the

about 3 times the linear drop of the first cycle. Thus, a good

place to start is with the output capacitor size of approxi-

mately:

C

I

fV

OUT

OUT

O

DROOP

≈

∆

3

More capacitance may be required depending on the duty

cycle and load step requirements.

In most applications, the input capacitor is merely re-

quired to supply high frequency bypassing, since the

impedance to the supply is very low. A 10

µF ceramic

capacitor is usually enough for these conditions.

Setting the Output Voltage

The KB3511 develops a 0.6V reference voltage between

Figure 2. The output voltage is set by a resistive divider

according to the following formula: