LTC1753CSW#TRPBF Datasheet LTC1753CSW#PBF, LTC1753CSW#TRPBF, LTC1753CG#TRPBF | P16-P18

LTC1753

1753fa

Note that capacitor manufacturers’ ripple current ratings

are often based on only 2000 hours (three months)

lifetime at rated temperature. Further derating of the input

capacitor ripple current beyond the manufacturer’s speci-

fication is recommended to extend the useful life of the

circuit. Lower operating temperature will have the largest

effect on capacitor longevity.

The output capacitor in a buck converter sees much less

ripple current under steady-state conditions than the input

capacitor. Peak-to-peak current is equal to that in the

inductor, usually 10% to 40% of the total load current.

Output capacitor duty places a premium not on power

dissipation but on ESR. During an output load transient,

the output capacitor must supply all of the additional load

current demanded by the load until the LTC1753 can

adjust the inductor current to the new value. Output

capacitor ESR results in a step in the output voltage equal

to the ESR value multiplied by the change in load current.

An 11A load step with a 0.05Ω ESR output capacitor will

result in a 550mV output voltage shift; this is 19.6% of the

output voltage for a 2.8V supply! Because of the strong

relationship between output capacitor ESR and output

load transient response, the output capacitor is usually

chosen for ESR, not for capacitance value; a capacitor with

suitable ESR will usually have a larger capacitance value

than is needed for energy storage.

Electrolytic capacitors rated for use in switching power

supplies with specified ripple current ratings and ESR can

be used effectively in LTC1753 applications. OS-CON

electrolytic capacitors from Sanyo and other manufactur-

ers give excellent performance and have a very high

performance/size ratio for electrolytic capacitors. Surface

mount applications can use either electrolytic or dry

tantalum capacitors. Tantalum capacitors must be surge

tested and specified for use in switching power supplies.

Low cost, generic tantalums are known to have very short

lives followed by explosive deaths in switching power

supply applications. AVX TPS series surface mount

devices are popular surge tested tantalum capacitors that

work well in LTC1753 applications.

A common way to lower ESR and raise ripple current

capability is to parallel several capacitors. A typical LTC1753

application might exhibit 5A input ripple current. Sanyo

OS-CON part number 10SA220M (220µF/10V) capacitors

feature 2.3A allowable ripple current at 85°C; three in

parallel at the input (to withstand the input ripple current)

will meet the above requirements. Similarly, AVX

TPSE337M006R0100 (330µF/6V) have a rated maximum

ESR of 0.1Ω; seven in parallel will lower the net output

capacitor ESR to 0.014Ω. For low cost application, Sanyo

MV-GX series of capacitors can be used with acceptable

performance. The small size, low profile Sanyo OS-CON

4SP820M comes with extremely low ESR (typically 0.008Ω

at room temperature). This is an excellent choice for

output capacitor usage. However, due to the low ESR, it

requires attention to frequency compensation. Refer to

the Feedback Loop Compensation section for details.

Feedback Loop Compensation

The LTC1753 voltage feedback loop is compensated at the

COMP pin, attached to the output node of the internal g

error amplifier. The feedback loop can generally be com-

pensated properly with an RC + C network from COMP to

GND as shown in Figure 7a.

Loop stability is affected by the values of the inductor,

output capacitor, output capacitor ESR, FET R

DS(ON)

, error

amplifier transconductance and error amplifier compen-

sation network. The inductor and the output capacitor

create a double pole at the frequency:

2π√(L

)(C

OUT

)

1753 F07a

DAC

LTC1753

SENSE

COMP

1µF

–

ERR

Figure 7a. Compensation Pin Hook-Up

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LTC1753

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Figure 7b. Bode Plot of the LTC1753 Overall Transfer Function

Figure 7c. Bode Plot of the LTC1753 Overall Transfer Function

Using a Low ESR Output Capacitor

–20dB/DECADE

LOOP GAIN

ESR

FREQUENCY

1753 F07b

= LTC1753 SWITCHING

FREQUENCY

= CLOSED-LOOP CROSSOVER

FREQUENCY

poor load transient response despite the improvement in

output voltage ripple.

To resolve this problem, a small capacitor can be con-

nected between the SENSE and V

pins to create a pole-

zero pair in the loop compensation. The zero location is

prior to the pole location and thus, phase lead can be

added to boost the phase margin at the loop crossover

frequency. The pole and zero locations are located at:

ZC2

= and

2π(R2)(C2)

PC2

2π(R12)(C2)

where R12 is the parallel combination resistance of R1 and

R2. Choose C2 so that the zero is located at a lower

frequency compared to f

and the pole location is high

enough that the closed loop has enough phase margin for

stability. Figure 7c shows the Bode plot using phase lead

compensation around the LTC1753 internal resistor

divider network.

Although a mathematical approach to frequency compen-

sation can be used, the added complication of input and/

or output filters, unknown capacitor ESR, and gross

operating point changes with input voltage, load current

variations, all suggest a more practical empirical method.

This can be done by injecting a transient current at the load

and using an RC network box to iterate toward the final

compensation values, or by obtaining the optimum loop

The ESR of the output capacitor forms a zero at the

frequency:

ESR

2π(ESR)(C

OUT

)

The compensation network at the error amplifier output is

to provide enough phase margin at the 0dB crossover

frequency for the overall closed-loop transfer function.

The zero and pole from the compensation network are:

2π(R

)(C

)

and

2π(R

)(C1)

respectively.

Figure 7b shows the Bode plot of the overall transfer

function.

The compensation value used in this design is based on

the following criteria: f

= 12f

, f

= f

and f

= 5f

. At

the loop crossover frequency f

, the attenuation due the

LC filter and the input resistor divider is compensated by

the gain of the PWM modulator and the gain of the error

amplifier (g

mERR

)(R

When low ESR output capacitors (Sanyo OS-CON) are

used, the ESR zero can be high enough in frequency that

it provides little phase boost at the loop crossover fre-

quency. Therefore, inadequate phase margin is obtained

for the system. This causes loop stability problems and

–20dB/DECADE

LOOP GAIN

ZC2

ESR

PC2

FREQUENCY

1753 F07c

= LTC1753 SWITCHING

FREQUENCY

= CLOSED-LOOP CROSSOVER

FREQUENCY

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LTC1753

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response using a network analyzer to find the actual loop

poles and zeros.

Table 5 shows the suggested compensation components

for 5V input applications based on the inductor and output

capacitor values. The values were calculated using mul-

tiple paralleled 330µF AVX TPS series surface mount

tantalum capacitors as the output capacitor. The optimum

component values might deviate from the suggested

values slightly because of board layout and operating

condition differences.

Table 5. Suggested Compensation Network for 5V Input

Application Using Multiple Paralleled 330µF AVX TPS Output

Capacitors

(

H) C

(

F) R

Ω

(

F) C1 (pF)

1 990 1.8 0.022 680

1 1980 3.6 0.01 330

1 4950 9.1 0.01 120

2.7 990 5.1 0.01 220

2.7 1980 10 0.01 120

2.7 4950 24 0.0047 47

5.6 990 10 0.01 120

5.6 1980 20 0.0047 56

5.6 4950 51 0.0033 22

An alternate output capacitor is the Sanyo MV-GX series.

Using multiple parallel 1500µF Sanyo MV-GX capacitors

for the output capacitor, Table 6 shows the suggested

compensation component value for a 5V input application

based on the inductor and output capacitor values.

Table 6. Suggested Compensation Network for 5V Input

Application Using Multiple Paralleled 1500µF Sanyo MV-GX

Output Capacitors

(

H) C

(

F) R

Ω

(

F) C1 (pF)

1 4500 4.3 0.022 270

1 6000 5.6 0.015 220

1 9000 8.2 0.01 150

2.7 4500 11 0.01 100

2.7 6000 15 0.01 82

2.7 9000 22 0.01 56

5.6 4500 24 0.01 56

5.6 6000 30 0.0047 39

5.6 9000 47 0.0047 27

Table 7 shows the suggested compensation component

value for a 5V application based on the Sanyo OS-CON

4SP820M low ESR output capacitors

Table 7. Suggested Compensation Network for 5V Input

Application Using Multiple Paralleled 820µF Sanyo OS-CON

4SP820M Output Capacitors

(

H) C

(

F) R

Ω

(

F) C1 (pF) C2 (pF)

1 1640 5.6 0.01 220 270

1 2460 9.1 0.0047 150 270

1 4100 15 0.0047 82 270

2.7 1640 16 0.0047 82 270

2.7 2460 24 0.0033 56 270

2.7 4100 39 0.0022 33 270

5.6 1640 33 0.0033 39 270

5.6 2460 47 0.0022 27 270

5.6 4100 82 0.0022 15 270

Remote Sense Considerations

In some installations such as Intel Slot 2 designs, the

regulator is by necessity a relatively long distance from the

load. It is desirable in these instances to connect the

regulator sense connection at the load rather than directly

at the regulator output. This forces the supply voltage to

be regulated at the load which, after all, is the desired point

to control. In most cases no problems will be encountered

as a result of doing this. However, care must be exercised

if the power path is long or the capacitance at the load is

very large.

The power distribution path has some finite amount of

inductance. There will also be a significant amount of

capacitance at the load as the local bypass. These two

circuit elements constitute a second order, lowpass filter

and the SENSE lead connects to the output of this filter. As

is true for any LC filter, there is 180° of phase shift at a

frequency beyond the double pole. If the resonant fre-

quency of the filter falls below the regulator’s feedback

loop crossover frequency, the loop will likely oscillate.

There are a couple of measures that may be taken to

alleviate this problem. The first is to minimize the induc-

tance of the power path. Therefore, it is desirable to make

the power trace as wide as possible and as short as

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Switching Voltage Regulators 5-B Progmable Sync Sw Reg Cntr for Penti

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