LTC1772B データシートの表示（PDF） - Linear Technology

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LTC1772B

Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23

Linear Technology 

LTC1772B

APPLICATIONS INFORMATION

where PD is the allowable power dissipation and will be

determined by efficiency and/or thermal requirements.

A fast switching diode must also be used to optimize

efficiency. Schottky diodes are a good choice for low

forward drop and fast switching times. Remember to keep

lead length short and observe proper grounding (see

Board Layout Checklist) to avoid ringing and increased

dissipation.

CIN and COUT Selection

In continuous mode, the source current of the P-channel

MOSFET is a square wave of duty cycle (VOUT + VD)/

(VIN + VD). To prevent large voltage transients, a low ESR

input capacitor sized for the maximum RMS current must

be used. The maximum RMS capacitor current is given by:

[ ( )]1/2

VOUT VIN − VOUT

CIN Required IRMS ≈ IMAX

VIN

This formula has a maximum value at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even significant deviations

do not offer much relief. Note that capacitor manufacturer’s

ripple current ratings are often based on 2000 hours of life.

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may be paralleled to meet the

size or height requirements in the design. Due to the high

operating frequency of the LTC1772B, ceramic capacitors

can also be used for CIN. Always consult the manufacturer

if there is any question.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically, once the ESR require-

ment is satisfied, the capacitance is adequate for filtering.

The output ripple (∆VOUT) is approximated by:

∆VOUT

≈



IRIPPLE ESR

+

1

4fC OUT





where f is the operating frequency, COUT is the output

capacitance and IRIPPLE is the ripple current in the induc-

tor. The output ripple is highest at maximum input voltage

since ∆IL increases with input voltage.

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. Once the ESR requirement for COUT has been

met, the RMS current rating generally far exceeds the

IRIPPLE(P-P) requirement.

Low Supply Operation

Although the LTC1772B can function down to approxi-

mately 2.0V, the maximum allowable output current is

reduced when VIN decreases below 3V. Figure 4 shows the

amount of change as the supply is reduced down to 2V.

Also shown in Figure 4 is the effect of VIN on VREF as VIN

goes below 2.3V.

105

VREF

100

95

VITH

90

85

80

75

2.0

2.2

2.4

2.6

2.8

3.0

INPUT VOLTAGE (V)

1772 F03

Figure 4. Line Regulation of VREF and VITH

8

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