7

LTC1754-3.3/LTC1754-5

range. The capacitor manufacturer’s data sheet should be

consulted to determine what style and value of capacitor

is needed to ensure 0.6

µF at all temperatures.

Output Ripple

Low frequency

regulation mode ripple exists due to the

hysteresis in the sense comparator and propagation delay

in the charge pump control circuit. The amplitude and

frequency of this ripple are heavily dependent on the load

current, the input voltage and the output capacitor size.

because the increased strength of the charge pump causes

fast edges that may outpace the regulation circuitry.

Generally the regulation ripple has a sawtooth shape

associated with it.

A high frequency ripple component may also be present

on the output capacitor due to the charge transfer action

of the charge pump. In this case the output can display a

voltage pulse during the charging phase. This pulse

results from the product of the charging current and the

ESR of the output capacitor. It is proportional to the input

voltage, the value of the flying capacitor and the ESR of the

output capacitor.

Typical combined output ripple for the LTC1754-5 with

10

µF output capacitor. A smaller output capacitor and/or

larger output current load will result in higher ripple due to

higher output voltage slew rates.

There are several ways to reduce output voltage ripple. For

mended. A larger capacitor will reduce both the low and

high frequency ripple due to the lower charging and

discharging slew rates, as well as the lower ESR typically

found with higher value (larger case size) capacitors. A low

ESR ceramic output capacitor will minimize the high

frequency ripple, but will not reduce the low frequency

ripple unless a high capacitance value is used. To reduce

both the low and high frequency ripple, a reasonable

compromise is to use a 10

µF to 22µF tantalum capacitor

in parallel with a 1

An R-C filter may also be used to reduce high frequency

voltage spikes (see Figure 1).

Figure 1. Output Ripple Reduction Techniques

flying capacitor may be used to reduce output ripple. A

smaller flying capacitor (0.01

µF to 0.47µF) delivers less

charge per clock cycle to the output capacitor resulting in

lower output ripple. However, with a smaller flying capaci-

tor, the maximum available output current will be reduced

along with the efficiency.

Note that when using a larger output capacitor the turn on

time of the device will increase.

Inrush Currents

sients in the 50mA to 100mA range whenever the charge

pump is enabled. However during start-up, inrush cur-

rents may approach 250mA. For this reason it is important

to minimize the source impedance between the input

result in regulation problems or prevent start-up.

Ultralow Quiescent Current Regulated Supply

The LTC1754 contains an internal resistor divider (refer to

the Simplified Block Diagram) that typically draws 1.5

µA

cycle signal to the SHDN pin ensures that the circuit of

Figure 2 comes out of shutdown frequently enough to

maintain regulation. Since the LTC1754 spends nearly the

entire time in shutdown, the no-load quiescent current is

The LTC1754 must be out of shutdown for a minimum

duration of 200

µstoallowenoughtimetosensetheoutput

voltage and keep it in regulation. A 2Hz, 2% duty cycle

LTC1754-X

15

µF

TANTALUM

1

µF

CERAMIC

LTC1754-X

2

Ω

10

µF

TANTALUM

10

µF

TANTALUM

1754 F01

+

+

+

APPLICATIO S I FOR ATIO