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Data Sheet

ADP5600

Rev. 0 | Page 23 of 25

DESIGN EXAMPLE

This section provides an example of the step by step design

procedures and the external components required for ADP5600.

Table 9 lists the design requirements for this example.

Table 9. Example Design Requirements for ADP5600

Parameter

Specification

LDO Output Voltage

VLDO_OUT = −3.3 V

LDO Output Current

ILDO_OUT = −100 mA

SETTING THE SWITCHING FREQUENCY OF THE

CHARGE PUMP

The first step is to determine the switching frequency for the

ADP5600 design. In general, higher switching frequencies

produce a smaller solution size due to the lower component

values required, whereas lower switching frequencies result in

higher conversion efficiency due to lower switching losses.

Figure 63. Power Efficiency vs. ICPOUT at Various Oscillator Frequencies

The oscillator frequency of the ADP5600 can be set from

0.1 MHz to 1 MHz by connecting a resistor from the FREQ pin

to ground. The selected resistor allows the user to make decisions

based on the trade-off between efficiency and solution size.

In this design example, a switching frequency of 500 kHz

achieves an ideal combination of small solution size and high

conversion efficiency. To set the switching frequency to 500 kHz,

use the following equation to calculate the RT value:

RT [kΩ] = 64,700/fOSC [kHz]

Therefore, select a standard resistor, RT = 130 kΩ.

SELECTING THE CHARGE PUMP FLYING CAPACITOR

The flying capacitor dictates the amount of voltage drop across

the charge pump due to the output resistance ,which depends

on the charge pump switching frequency.

Operation at high switching frequencies allows the use of

smaller flying capacitances, however, the minimum value is

limited due to its inverse effect on the charge pump impedance.

Refer to Table 10 for the recommended flying capacitor value for

each switching frequency.

Table 10. Recommended Minimum C1 and C2

fOSC

C1 and C2 Capacitances

100 kHz

1 µF

250 kHz

1 µF

500 kHz

1 µF

750 kHz

0.47 µF

1 MHz

0.47 µF

SETTING THE OUTPUT VOLTAGE OF THE LDO

REGULATOR

Select a value for R2 and then calculate R1 by using the following

equation:

R1 = ((VADJ/VLDO_OUT) −1) × R2

where:

VLDO_OUT is −2.5 V.

R1 is the feedback resistor between LDO_OUT and FB.

R2 is the feedback resistor between FB and GND (R2 is

recommended to be 40 kΩ or higher).

To set the output voltage to −3.3 V, R1 is set to 40 kΩ, giving a

calculated R2 value of 155.9 kΩ.

DETERMINING THE MINIMUM VIN VOLTAGE

To achieve the desired performance of the ADP5600, a

minimum input voltage, VIN, is required per application. This

both considers the PSRR performance that requires a headroom

voltage across the LDO and the drop on the charge pump due to

the output resistance. To calculate the minimum VIN, use the

following formula:

VIN = VLDO_OUT + VHR + (ROUT × ICPOUT)

where:

ROUT is the output resistance of the charge pump.

VHR is the LDO headroom required to achieve a certain PSRR

performance. The recommended minimum headroom voltage

is 500 mV.

CPOUT LOAD CURRENT (mA)

fOSC = 100kHz

fOSC = 500kHz

fOSC = 1MHz

100

–100

–90

–30

–70

–20

–50

–80

–60

–40

–10

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