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

U

OPERATIO (Refer to Functional Diagram)

Overvoltage Protection

As a further protection, the overvoltage comparator in the

LTC1772B will turn the external MOSFET off when the

feedback voltage has risen 7.5% above the reference

voltage of 0.8V. This comparator has a typical hysteresis

of 20mV.

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

( ) IPK

=

VITH – 0.85

10 RSENSE

when the LTC1772B is operating below 40% duty cycle.

However, once the duty cycle exceeds 40%, slope

compensation begins and effectively reduces the peak

inductor current. The amount of reduction is given by the

curves in Figure 3.

110

100

90

80

70

60

50

IRIPPLE = 0.4IPK

40

AT 5% DUTY CYCLE

30

IRIPPLE = 0.2IPK

AT 5% DUTY CYCLE

20 VIN = 4.2V

10

0 10 20 30 40 50 60 70 80 90 100

DUTY CYCLE (%)

1772 F03

Figure 3. Maximum Output Current vs Duty Cycle

APPLICATIONS INFORMATION

The basic LTC1772B application circuit is shown

in␣ Figure␣ 1. External component selection is driven by the

load requirement and begins with the selection of L1 and

RSENSE (= R1). Next, the power MOSFET, M1 and the

output diode D1 is selected followed by CIN (= C1)and

COUT(= C2).

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

With the current comparator monitoring the voltage devel-

oped across RSENSE, the threshold of the comparator

determines the inductor’s peak current. The output cur-

rent the LTC1772B can provide is given by:

IOUT

=

0.105

RSENSE

−

IRIPPLE

2

where IRIPPLE is the inductor peak-to-peak ripple current

(see Inductor Value Calculation section).

A reasonable starting point for setting ripple current is

IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it

becomes:

RSENSE =

0.0875

IOUT

for Duty Cycle

<

40%

6

However, for operation that is above 40% duty cycle, slope

compensation effect has to be taken into consideration to

select the appropriate value to provide the required amount

of current. Using Figure 3, the value of RSENSE is:

( ) RSENSE = (0.0875)SF

IOUT 100

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the use

of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses.

The inductance value also has a direct effect on ripple

current. The ripple current, IRIPPLE, decreases with higher

inductance or frequency and increases with higher VIN or

VOUT. The inductor’s peak-to-peak ripple current is given

by:

( ) IRIPPLE

=

VIN

− VOUT

fL





VOUT + VD 

VIN + VD 

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