7

LTC1550/LTC1551

except that the Shutdown pin is active-high (SHDN). All

members of the LTC1550 family feature a 900kHz charge

pump frequency. The LTC1550/ LTC1551 come standard

with fixed – 4.1V output voltages and the LTC1550 is

available with an adjustable output voltage. Both devices

can be configured with other fixed output voltages; contact

Linear Technology for more information.

The LTC1550 consists of two major blocks (see Block

Diagram): an inverting charge pump and a negative linear

regulator. The charge pump uses two external capacitors,

operates by charging and discharging C1 on alternate

phases of the internal 900kHz clock. C1 is initially charged

clock changes phase, S1 and S3 open and S2 and S4 close,

shorting the positive side of C1 to ground. This forces the

negative side of C1 below ground, and charge is transferred

clock frequency helps keep noise out of 400kHz to 600kHz

IF bands commonly used by portable radio frequency

systems and reduces the size of the external capacitors

required. Most applications can use standard 0.1

µFceramic

0.1

µF has little effect on the output ripple or the output

current capacity of the LTC1550/LTC1551.

negative regulator block. This block consists of an

N-channel MOSFET pass device and a feedback amplifier

that monitors the output voltage and compares it to the

internal reference. The regulated output appears at the

sient response, enabling it to remove most of the switch-

and 10mA. The output voltage is set to – 4.1V by a pair of

internal divider resistors. The N-channel pass device mini-

mizes dropout, allowing the output to remain in regulation

with supply voltages as low as 4.5V. An output capacitor

of at least 4.7

the regulator loop stable; for optimum stability and mini-

mum output ripple, at least 10

µF is recommended.

6, 7

10

PGND, AGND

LTC1550

4, 11

ADJ

R1

R2

R1 + R2

R2

()

LTC1550/51 • F02

Figure 1. External Resistor Connections

Adjustable Hook-Up

The LTC1550CGN is available in an adjustable output

version in a 16-pin SSOP package. The output voltage is

(Figure 1). Note that the internal reference and the internal

feedback amplifier are set up as a positive-output regula-

tor referenced to the SENSE pin, not a negative regulator

referenced to ground. The output resistor divider must be

For example, a – 3V output would require a 13k resistor

after connecting the divider resistors, the output voltage is

not what you expected, try swapping them.

CAPACITOR SELECTION

The LTC1550/LTC1551 requires four external capacitors:

an input bypass capacitor, two 0.1

µF charge pump capaci-

tors and an output filter capacitor. The overall behavior of

the LTC1550/LTC1551 is strongly affected by the capaci-

tors used. In particular, the output capacitor has a signifi-

cant effect on the output ripple and noise performance.

Proper capacitor selection is critical for optimum perfor-

mance of the LTC1550/LTC1551.

Output Ripple vs Output Capacitor

Figure 3 shows the effect of using different output capaci-

tor values on LTC1550/LTC1551 output ripple. These

curves are taken using the circuit in Figure 2, with

performance with a standard tantalum capacitor alone and

the lower curve shows the tantalum capacitor in parallel

with a 0.1

µF ceramic capacitor. As a general rule, larger