Application Information

PRE-REGULATION

The very low input current ripple of the LM3570, resulting

from internal pre-regulation, adds very little noise to the input

line. The core of the LM3570 is very similar to that of a basic

3/2 switched capacitor regulator: it is composed of seven

switches and two flying capacitors (external). Regulation is

achieved by modulating the on-resistance of the three

switches connected to the input pin (one switch in phase one

and two in phase two). The regulation is done before the

voltage “gain stage”, giving rise to the term "pre-regulation".

Pre-regulation eliminates most of the input current ripple that

is a typical and undesirable characteristic of many switched

capacitor converters.

TOTAL OUTPUT CURRENT CAPABILITY

The LM3570 is capable of providing a total output current of

80mA. The 80mA can be divided through any combination of

the three dedicated current outputs and/or current drawn

from the V

pin, the LM3570 will hold the output voltage at the regulated

4.35V. This pre-regulation occurs when the input voltage is

within the 3.0V to 5.5V operating range. If the input voltage is

between the 2.7V to 3.0V range, the V

have in the same manner as the output of an unregulated

charge pump. During operation in this input voltage range,

the output voltage becomes directly related to the total out-

put current drawn from the part and the output resistance

(R

LM3570’s R

V

where 2.7V

≤ V

Ω

Dx OUTPUT CURRENT CAPABILITY

An external resistor, R

the current to be mirrored in each of the three LED outputs.

The internal current mirror sets each LED output current with

a 100:1 ratio to the current through R

circuitry matches the current through each LED to within

0.5%. An equation for approximating the LED current is:

R

PARALLEL Dx OUTPUTS FOR INCREASED CURRENT

CAPABILITY

Outputs D

combination to drive higher currents through fewer LEDs.

For example in Figure 2, outputs D

together to drive one LED while D

ate LED.

With this configuration, two parallel current sources of equal

value provide current to one of the LEDs. R

therefore be chosen so that the current through each output

is programmed to 50% of the desired current through the

parallel connected LED. For example, if 40mA is the desired

drive current for the parallel connected LED, R

selected so that the current through each of the outputs is

20mA. Other combinations of parallel outputs may be imple-

mented in similar fashions, such as in Figure 3.

Connecting outputs in parallel does not affect internal opera-

tion of the LM3570 and has no impact on the Electrical

20085004

FIGURE 1. LM3570 Charge Pump Model

20085007

FIGURE 2. One Parallel Connected and One Singular

Connected LED

20085006

FIGURE 3. One Parallel Connected LED

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