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LT1500

Adaptive-Frequency Current Mode Switching Regulators

Linear Technology 

LT1500/LT1501

APPLICATIONS INFORMATION

type TPS from AVX, or type 595D from Sprague. These

have an ESR of 0.06Ω in a “E” case size. At lower output

current levels, a 100µF unit in a “D” case size may be

sufficient. Output ripple voltage can be calculated from:

VRIPPLE

=

ESR

0.1+



1.2(IOUT )(VOUT

VIN

)





Loop frequency stability is affected by the characteristics

of the output capacitor. The ESR of the capacitor should be

very low, and the capacitance must be large (> 200µF) to

ensure good loop stability under worst-case conditions of

low input voltage, higher output voltages, and high load

currents. The 14-pin LT1500 can use external frequency

compensation on the VC pin to give good loop stability with

smaller output capacitors. See Loop Stability section for

details.

Precautions regarding solid tantalum capacitors for input

bypassing do not apply to the output capacitor because

turn-on surges are limited by the inductor and discharge

surges do not harm the capacitors.

Setting Output Voltage

Preset 3.3V and 5V parts are available. For other voltage

applications the adjustable part uses an external resistor

divider to set output voltage. Bias current for the feedback

(FB) pin is typically ±30nA (it is internally compensated).

Thevenin divider resistance should be 100kΩ or less to

keep bias current errors to a minimum. This leads to a

value for R1 and R2 (see Figure 1) of:

R1= 100kΩ(VOUT )

1.265V

R2 = R1(1.265)

VOUT – 1.265

Example: VOUT = xxV

R1

=

100kΩ(12)

1.265

= 949k

(use 1M)

R2 = 1M(1.265) = 118k

12 – 1.265

Note that there is an internal switch that disconnects the

internal divider for fixed 3.3V and 5V parts in shutdown.

This prevents the divider from adding to shutdown cur-

rent. Without this switch, shutdown current increases

because of the divider current directly, but even more so

if the FB pin is held above 0.6V by the divider. See graphs

in Typical Performance Characteristics.

VOUT = 12V

R1

1M

FB

1%

ERROR

AMPLIFIER

1.265V

R2

118K

1%

LTC1500/01 • F01

Figure 1. External Voltage Divider

Selectable Output (Fixed Voltage Parts)

The Select pin (available only on LT1500-3/5) allows the

user to select either a 3.3V or 5V output. Floating the pin

sets output voltage at 3.3V and grounding the pin sets

output voltage at 5V. The equivalent circuit of the Select

pin function is shown in Figure 2.

VOUT

204k

ERROR

AMPLIFIER

69k

SELECT

1.265V

58k

GND

LTC1500/01 • F02

Figure 2. Schematic of Select Pin Function

Note that there is a switch in series with the VOUT pin. This

switch is turned off in shutdown to eliminate shutdown

current drawn by the voltage divider. For adjustable parts

11

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