ADP1883ARMZ-1.0-R7 datasheet(20/40 Pages) AD

20 / 40 page

ADP1882/ADP1883

Rev. 0 | Page 20 of 40

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 a soft start begins. It

forces a 0.4 V reference value at the DRVL output (see Figure 68)

and is programmed to identify four possible resistor values:

47 kΩ, 22 kΩ, open, and 100 kΩ.

DRVH

DRVL

RES

ADP1882

CS GAIN

PROGRAMMING

Figure 68. Programming Resistor Location

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

DRVL 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

corresponds to a current-sense gain (ACS) of 3 V/V, 6 V/V, 12 V/V,

and 24 V/V, respectively (see Table 5 and Table 6). This variable

is used for the valley current-limit setting, which sets up the

appropriate current-sense signal gain for a given application

and sets the compensation necessary to achieve loop stability

(see the Valley Current-Limit Setting and Compensation

Network sections).

PGND

CS GAIN SET

AMP

ADC

DRVL

RES

0.4V

Figure 69. RES Detect Circuit for Current-Sense Gain Programming

Table 5. Current-Sense Gain Programming

Resistor (kΩ)

ACS (V/V)

3.25

6.5

Open

100

VALLEY CURRENT-LIMIT SETTING

The architecture of the ADP1882/ADP1883 is based on valley

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

components: the RON of the lower-side MOSFET, the error ampli-

fier output voltage swing (COMP), and the current-sense gain.

The COMP range is internally fixed at 1.5 V. The current-sense

gain is programmable via an external resistor at the DRVL pin

(see the Programming Resistor (RES) Detect Circuit section).

The RON of the lower-side MOSFET can vary over temperature

and usually has a positive TC (meaning that it increases with

temperature); therefore, it is recommended that the current-

sense gain resistor be programmed based on the rated RON of

the MOSFET at 125°C.

Because the ADP1882/ADP1883 are based on valley current

control, the relationship between ICLIM and ILOAD is as follows:

ICLIM = ILOAD ×

⎟

⎠

⎞

−

⎜

⎝

⎛

where:

ICLIM is the desired valley current limit.

ILOAD is the current load.

KI is the ratio between the inductor ripple current and the

desired average load current (see Figure 10).

Establishing KI helps to determine the inductor value (see the

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

RIPPLE CURRENT =

ILOAD

LOAD CURRENT

VALLEY CURRENT LIMIT

Figure 70. Valley Current Limit to Average Current Relation

When the desired valley current limit (ICLIM) has been determined,

the current-sense gain can be calculated by using the following

expression:

ICLIM =

A ×

where:

ACS is the current-sense gain multiplier (see Table 5 and Table 6).

RON is the channel impedance of the lower-side MOSFET.

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