LTC1772HS6 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1772HS6

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

Linear Technology 

LTC1772

U

OPERATIO (Refer to Functional Diagram)

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

( ) IPK

=

VITH – 0.7

10 RSENSE

when the LTC1772 is operating below 40% duty cycle.

However, once the duty cycle exceeds 40%, slope com-

pensation begins and effectively reduces the peak induc-

tor current. The amount of reduction is given by the curves

in Figure 2.

110

100

90

80

70

60

50

40

IRIPPLE = 0.4IPK

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 F02

Figure 2. Maximum Output Current vs Duty Cycle

APPLICATIONS INFORMATION

The basic LTC1772 application circuit is shown in Figure 1.

External component selection is driven by the load re-

quirement and begins with the selection of L1 and RSENSE

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

D1 are 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 LTC1772 can provide is given by:

IOUT

=

0.12

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 =

1 for Duty Cycle

(10)(IOUT )

< 40%

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 2, the value of RSENSE is:

RSENSE

=

SF

(10)(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

f(L)

⎛

⎝⎜

VOUT + VD

VIN + VD

⎞

⎠⎟

1772fb

6

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