Datasheet

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
Q1
SW
Q2
RES
ADP1882
CS GAIN
PROGRAMMING
08901-067
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 (A
CS
) 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).
SW
PGND
CS GAIN SET
CS
AMP
ADC
DRVL
RES
0.4V
08901-068
Figure 69. RES Detect Circuit for Current-Sense Gain Programming
Table 5. Current-Sense Gain Programming
Resistor (kΩ) A
CS
(V/V)
47 3.25
22 6.5
Open 26
100 13
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 R
ON
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 R
ON
of the lower-side MOSFET can vary over temperature
and usually has a positive T
C
(meaning that it increases with
temperature); therefore, it is recommended that the current-
sense gain resistor be programmed based on the rated R
ON
of
the MOSFET at 125°C.
Because the ADP1882/ADP1883 are based on valley current
control, the relationship between I
CLIM
and I
LOAD
is as follows:
I
CLIM
= I
LOAD
×
⎟
⎠
⎞
−
2
1
I
K
⎜
⎝
⎛
where:
I
CLIM
is the desired valley current limit.
I
LOAD
is the current load.
K
I
is the ratio between the inductor ripple current and the
desired average load current (see Figure 10).
Establishing K
I
helps to determine the inductor value (see the
Inductor Selection section), but in most cases, K
I
= 0.33.
RIPPLE CURRENT =
I
LOAD
3
LOAD CURRENT
VALLEY CURRENT LIMIT
8901-069
0
Figure 70. Valley Current Limit to Average Current Relation
When the desired valley current limit (I
CLIM
) has been determined,
the current-sense gain can be calculated by using the following
expression:
I
CLIM
=
ONCS
RA ×
V5.1
where:
A
CS
is the current-sense gain multiplier (see Table 5 and Table 6).
R
ON
is the channel impedance of the lower-side MOSFET.