LT1576 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT1576

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1576/LT1576-5

APPLICATIONS INFORMATION

with large inductors. Output ripple voltage for the positive-

to-negative converter will be much higher than a buck

converter. Ripple current in the output capacitor will also

be much higher. The following equations can be used to

calculate operating conditions for the positive-to-negative

converter.

Maximum load current:

(( ( )( )())( ))(( )( ) ) IMAX

=



IP

−

2

VIN VOUT





VOUT + VIN f L 

VOUT + VIN − 0.35

VOUT

VOUT

VIN

+ VF

− 0.35

IP = Maximum rated switch current

VIN = Minimum input voltage

VOUT = Output voltage

VF = Catch diode forward voltage

0.35 = Switch voltage drop at 1.5A

Example: with VIN(MIN) = 5.5V, VOUT = 5V, L = 30µH,

VF = 0.5V, IP = 1.5A: IMAX = 0.6A. Note that this equation

does not take into account that maximum rated switch

current (IP) on the LT1576 is reduced slightly for duty

cycles above 50%. If duty cycle is expected to exceed 50%

(input voltage less than output voltage), use the actual IP

value from the Electrical Characteristics table.

Operating duty cycle:

DC =

VOUT + VF

VIN − 0.3 + VOUT + VF

(This formula uses an average value for switch loss, so it

may be several percent in error.)

With the conditions above:

DC =

5 + 0.5

= 51%

5.5 − 0.3 + 5 + 0.5

This duty cycle is close enough to 50% that IP can be

assumed to be 1.5A.

OUTPUT DIVIDER

If the adjustable part is used, the resistor connected to

VOUT (R2) should be set to approximately 5k. R1 is

calculated from:

( ) R1= R2 VOUT − 1.21

1.21

INDUCTOR VALUE

Unlike buck converters, positive-to-negative converters

cannot use large inductor values to reduce output ripple

voltage. At 200kHz, values larger than 75µH make almost

no change in output ripple. The graph in Figure 16 shows

peak-to-peak output ripple voltage for a 5V to – 5V con-

verter versus inductor value. The criteria for choosing the

150

5V TO –5V CONVERTER

OUTPUT CAPACITOR’S

120

ESR = 0.1Ω

DISCONTINUOUS

90

ILOAD = 0.1A

DISCONTINUOUS

60

ILOAD = 0.25A

30

CONTINUOUS

ILOAD > 0.38A

0

0

15

30

45

60

75

INDUCTOR SIZE (µH)

1576 F16

Figure 16. Ripple Voltage on Positive-to-Negative Converter

24

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