ADP1878ACPZ-0.6-R7 datasheet(19/40 Pages) AD

19 / 40 page

Data Sheet

ADP1878/ADP1879

Rev. A | Page 19 of 40

Table 5. Power Input and LDO Output Configurations

VIN

VREG

Comments

>5.5 V

Float

Must use the LDO

<5.5 V

Connect to VIN

LDO drop voltage is not

realized (that is, if VIN = 2.75 V,

then VREG = 2.75 V)

<5.5 V

Float

LDO drop is realized

VIN ranging

above and

below 5.5 V

Float

LDO drop is realized, minimum

VIN recommendation is 2.95 V

THERMAL SHUTDOWN

Thermal shutdown is a protection feature that prevents the IC

from damage caused by a very high operating junction temper-

ature. If the junction temperature of the device exceeds 155°C,

the device enters the thermal shutdown state. In this state, the

device shuts off both the high- and low-side MOSFETs and disables

the entire controller immediately, thus reducing the power con-

sumption of the IC. The device resumes operation after the

junction temperature of the device cools to less than 140°C.

PROGRAMMING RESISTOR (RES) DETECT CIRCUIT

Upon startup, one of the first blocks to become active is the RES

detect circuit. This block powers up before soft start begins. It

forces a 0.4 V reference value at the RES pin (see Figure 68) and is

programmed to identify four possible resistor values: 47 kΩ, 22 kΩ,

open, and 100 kΩ.

The RES detect circuit digitizes the value of the resistor at the

RES pin (Pin 6). An internal ADC outputs a 2-bit digital code

that is used to program four separate gain configurations in the

current sense amplifier (see Figure 69). Each configuration corre-

sponds to a current sense gain (ACS) of 3 V/V, 6 V/V, 12 V/V, or

24 V/V, respectively (see Table 6 and Table 7). This variable is used

for the valley current-limit setting, which sets up the appropriate

current sense gain for a given application and sets the compensation

necessary to achieve loop stability (see the Valley Current-Limit

Setting section and the Compensation Network section).

Figure 68. Programming Resistor Location

Figure 69. RES Detect Circuit for Current Sense Gain Programming

Table 6. Current Sense Gain Programming

Resistor

ACS

47 kΩ

3 V/V

22 kΩ

6 V/V

Open

12 V/V

100 kΩ

24 V/V

VALLEY CURRENT-LIMIT SETTING

The architecture of the ADP1878/ADP1879 is based on valley

current-mode control. The current limit is determined by three

components: the RON of the low-side MOSFET, the output voltage

swing of the current sense amplifier, and the current sense gain.

The output range of the current sense amplifier is internally

fixed at 1.4 V. The current sense gain is programmable via an

external resistor at the RES pin (see the Programming Resistor

(RES) Detect Circuit section). The RON of the low-side MOSFET

can vary over temperature and usually has a positive TC (meaning

that it increases with temperature); therefore, it is recommended to

program the current sense, gain resistor based on the rated RON of

the MOSFET at 125°C.

Because the ADP1878/ADP1879 are based on valley current

control, the relationship between ICLIM and ILOAD is

where:

KI is the ratio between the inductor ripple current and the

desired average load current (see Figure 70).

ICLIM is the desired valley current limit.

ILOAD is the current load.

Establishing KI helps to determine the inductor value (see the

Inductor Selection section), but in most cases, KI = 0.33.

Figure 70. Valley Current Limit to Average Current Relation

DRVH

DRVL

RES

CS GAIN

PROGRAMMING

PGND

CS GAIN

SET

AMP

ADC

RES

0.4V

LOAD CURRENT

VALLEY CURRENT LIMIT

RIPPLE CURRENT =

ILOAD

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