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LTC1700EMS

No RSENSE Synchronous Step-Up DC/DC Controller

Linear Technology 

LTC1700

APPLICATIONS INFORMATION

preferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard”, which means that

inductance collapses abruptly when the peak design cur-

rent is exceeded. This results in an abrupt increase in

inductor ripple current and output voltage ripple. Do not

allow the core to saturate!

Molypermalloy (from Magnetics, Inc.) is a very good, low

loss core material for toroids, but it is more expensive than

ferrite. A reasonable compromise from the same manu-

facturer is Kool Mµ. Toroids are very space efficient,

especially when you can use several layers of wire. Be-

cause they generally lack a bobbin, mounting is more

difficult. However, new designs for surface mount are

available which do not increase the height significantly.

COUT Selection

During continuous operation, the output capacitor has a

trapezoidal current profile. The RMS current into the

capacitor is then given by:

⎛

ICOUT(RMS) ≅ ⎝⎜IOUT

VOUT

VIN

⎞

– 1⎠⎟

The RMS current is greatest at IOUT(MAX) and minimum

input working voltage. Therefore the output capacitor

should be chosen with a rating at least ICOUT(RMS). Several

capacitors can also be paralleled to meet this requirement.

Besides RMS current rating, the selection of COUT is also

driven by the required effective series resistance (ESR).

The ESR of the capacitor together with its capacitance

determines the output ripple voltage and can be expressed

as:

( ) ∆VOUT

≈ IPK

ESR

+ 2IOUT

C OUT

tON

where COUT = output capacitance, tON = on time of main

MOSFET and IPK = peak inductor current. A common

technique to lower the total ESR at the output is to parallel

the output capacitor with a 10µF ceramic capacitor.

The choice of using a smaller output capacitance in-

creases the output ripple voltage due to the frequency

dependent term but can be compensated for by using

capacitors of very low ESR to maintain low ripple voltage.

The ITH pin OPTI-LOOP compensation components can be

optimized to provide stable, high performance transient

response regardless of the output capacitors selected.

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest ESR (size)

product of any aluminum electrolytic at a somewhat

higher price.

Multiple capacitors may have to be paralleled to meet the

ESR or RMS current handling requirements of the applica-

tion. Aluminum electrolytic and dry tantalum capacitors

are both available in surface mount configurations. In the

case of tantalum, 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 tantalum,

available in case heights ranging from 2mm to 4mm. Other

capacitor types include Sanyo OS-CON, Nichicon PL se-

ries and Sprague 593D and 595D series. Consult the

manufacturer for other specific recommendations.

Setting Output Voltage

The LTC1700 develops a 1.205V reference voltage be-

tween the feedback (Pin 3) terminal and ground (see

Figure 6). By selecting resistor R1, a constant current is

caused to flow through R1 and R2 to set the overall output

voltage. The regulated output voltage is determined by:

VOUT = 1.205(1 + R2/R1)

For most applications, a 30k resistor is suggested for R1.

To prevent stray pickup, a 100pF capacitor is suggested

across R1 located close to LTC1700.

Efficiency Considerations

LTC1700

VFB

VOUT

R2

R1

1700 • F06

Figure 6. Setting Output Voltage

1700fa

11

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