ADP1706/ADP1707/ADP1708

Rev. 0 | Page 12 of 20

APPLICATION INFORMATION

CAPACITOR SELECTION

Output Capacitor

The ADP1706/ADP1707/ADP1708 are designed for operation

with small, space-saving ceramic capacitors, but they function

with most commonly used capacitors as long as care is taken with

the effective series resistance (ESR) value. The ESR of the output

capacitor affects stability of the LDO control loop. A minimum of

4.7 μF capacitance with an ESR of 500 mΩ or less is recommended

to ensure stability of the ADP1706/ADP1707/ADP1708. Transient

response to changes in load current is also affected by output

capacitance. Using a larger value of output capacitance improves

the transient response of the ADP1706/ADP1707/ADP1708 to

large changes in load current. Figure 33 and Figure 34 show the

transient responses for output capacitance values of 4.7 μF and

22 μF, respectively.

TIME (2μs/DIV)

FROM 50mA TO 950mA

TIME (2μs/DIV)

FROM 50mA TO 950mA

Input Bypass Capacitor

Connecting a 4.7 μF capacitor from the IN pin to GND reduces

the circuit sensitivity to the printed circuit board (PCB) layout,

especially when long input traces, or high source impedance,

is encountered. If greater than 4.7 μF of output capacitance is

required, it is recommended that the input capacitor be increased

to match it.

Input and Output Capacitor Properties

Any good quality ceramic capacitors can be used with the

ADP1706/ADP1707/ADP1708, as long as they meet the

minimum capacitance and maximum ESR requirements.

Ceramic capacitors are manufactured with a variety of dielec-

trics, each with different behavior over temperature and applied

voltage. Capacitors must have a dielectric adequate to ensure

the minimum capacitance over the necessary temperature range

and dc bias conditions. X5R or X7R dielectrics with a voltage

rating of 6.3 V or 10 V are recommended. Y5V and Z5U

dielectrics are not recommended, due to their poor temperature

and dc bias characteristics.

VOLTAGE TRACKING APPLICATIONS

SS

TRK

IN

EN

IN

EN

OUT

OUT

RATIO VOLTAGE TRACKING

GND

GND

CORE RAIL

RATIO

TRACKING

I/O POWER RAIL

TIME

2.5V

1.2V

1.2V

2.5V

5V

R1

R2

ADP1706-2.5

ADP1707-1.2

Figure 35. Voltage Tracking Feature Using ADP1707

Figure 35 shows an application where the ADP1707 tracking

feature is used. An ADP1706 powers the I/O of a microproces-

sor and an ADP1707 powers the core. At startup, the output of

the ADP1706 ramps to 2.5 V, which is divided down via a

voltage divider (R1 and R2) to a lower voltage at the TRK pin of

the ADP1707. The output of the ADP1707 thus follows the

TRK pin and ramps up steadily to 1.2 V. This implementation

ensures that the core of the processor powers up after the I/O.

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP1706/ADP1707/ADP1708 are protected against

damage due to excessive power dissipation by current and

thermal overload protection circuits. The ADP1706/ADP1707/

ADP1708 are designed to reach current limit when the output

load reaches 1.5 A (typical). When the output load exceeds

1.5 A, the output voltage is reduced to maintain a constant

current limit.