Analog Devices ADP1878 Evaluation Board ADP1878-0.6-EVALZ ADP1878-0.6-EVALZ 数据表
产品代码
ADP1878-0.6-EVALZ
Data
Sheet
ADP1878/ADP1879
Rev. B | Page 19 of 40
Table 5. Power Input and LDO Output Configurations
VIN VREG Comments
>5.5 V
>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)
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
above and
below 5.5 V
Float
LDO drop is realized, minimum
VIN recommendation is 2.95 V
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.
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.
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 (A
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 (A
CS
) 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).
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 A
CS
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
is based on valley
current-mode control. The current limit is determined by three
components: the R
components: the R
ON
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 R
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 R
ON
of the low-side MOSFET
can vary over temperature and usually has a positive T
C
(meaning
that it increases with temperature); therefore, it is recommended to
program the current sense, gain resistor based on the rated R
program the current sense, gain resistor based on the rated R
ON
of
the MOSFET at 125°C.
Because the
are based on valley current
control, the relationship between I
CLIM
and I
LOAD
is
1
2
where:
K
K
I
is the ratio between the inductor ripple current and the
desired average load current (see Figure 70).
I
I
CLIM
is the desired valley current limit.
I
LOAD
is the current load.
Establishing K
I
helps to determine the inductor value (see the
Inductor Selection section), but in most cases, K
I
= 0.33.
Figure 70. Valley Current Limit to Average Current Relation
DRVH
DRVL
Q1
SW
Q2
RES
CS GAIN
PROGRAMMING
PROGRAMMING
094
41
-068
SW
PGND
CS GAIN
SET
CS
AMP
ADC
RES
0.4V
09
44
1-
0
69
LOAD CURRENT
VALLEY CURRENT LIMIT
RIPPLE CURRENT =
I
LOAD
3
09
44
1-
07
0