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LT1939EDD

Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller

Linear Technology 

LT1939

APPLICATIONS INFORMATION

The following example along with the data in Table 1

illustrates the tradeoffs of switch frequency selection.

Example.

VIN = 25V, VOUT1 = 3.3V, IOUT1 = 2A,

Temperature = 0°C to 85°C

tON(MIN) = 185ns (85°C from Typical Characteris-

tics

graph), VD = 0.6V, VSW = 0.4V (85°C)

Max

Frequency =

3.3 + 0.6

25
0.4 + 0.6

•

1

185ns

~ 835kHz

RT/SYNC ~ 49.9k

Frequency ≅ 820kHz

Input Voltage Range

Once the switching frequency has been determined, the

input voltage range of the regulator can be determined.

The minimum input voltage is determined by either the

LT1939’s minimum operating voltage of ~2.8V or by its

maximum duty cycle. The duty cycle is the fraction of time

that the internal switch is on during a clock cycle. The

maximum duty cycle can be determined from the clock

frequency and the minimum off time from the typical

characteristics graph.

This leads to a minimum input voltage of:

VIN(MIN)

=

VOUT1 + VD

DCMAX

VD

+

VSW

where VSW is the voltage drop of the internal switch,

and

DCMAX = 1 – tOFF(MIN) • Frequency.

Figure 3 shows a typical graph of minimum input voltage

vs load current for 3.3V and 5V applications.

The maximum input voltage is determined by the absolute

maximum ratings of the VIN and BST pins and by the

frequency and minimum duty cycle.

The minimum duty cycle is defined as:

DCMIN = tON(MIN) • Frequency

Maximum input voltage as:

VIN(MAX)

=

VOUT1 + VD

DCMIN

VD

+

VSW

8

FSW = 1MHz

L = 3.3μH

7

6

5

4

VOUT1 = 5V START-UP

3

VOUT1 = 5V RUNNING

VOUT1 = 3.3V START-UP

VOUT1 = 3.3V RUNNING

2

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

LOAD CURRENT (A)

1939 F03

Figure 3. Minimum Input Voltage vs Load Current

Table 1. Efficiency and Size Comparisons for Different RRT/SYNC Values, 3.3V Output

FREQUENCY

RT/SYNC

EFFICIENCY

VIN(MAX)

L

2.5MHz

15k

73.6

12

1μ

2.0MHz

20k

81.5

14

1.5μ

1.5MHz

24.9k

84.5

18

2.2μ

1.0MHz

40.2k

87.3

25

3.3μ

500kHz

90.9k

88.9

25

4.7μ

C

C + L AREA

(mm2)

10μ

24

10μ

24

10μ

24

22μ

34

47μ

40

1939f

11

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