ADP1879 Analog Devices, ADP1879 Datasheet - Page 19

• Manufacturer Part Number: ADP1879
• Description: Synchronous Buck Controller with Constant On-Time and Valley Current Mode with Power Saving Mode
• Manufacturer: Analog Devices

Data Sheet
Table 5. Power Input and LDO Output Configurations
VIN
>5.5 V
<5.5 V
<5.5 V
VIN ranging
above and
below 5.5 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 (A
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
RES
VREG
Float
Connect to VIN
Float
Float
CS GAIN
PROGRAMMING
DRVH
DRVL
SW
CS
Comments
Must use the LDO
LDO drop voltage is not
realized (that is, if VIN = 2.75 V,
then VREG = 2.75 V)
LDO drop is realized
LDO drop is realized, minimum
VIN recommendation is 2.95 V
) of 3 V/V, 6 V/V, 12 V/V, or
Q1
Q2
Rev. 0 | Page 19 of 40
Table 6. Current Sense Gain Programming
Resistor
47 kΩ
22 kΩ
Open
100 kΩ
VALLEY CURRENT-LIMIT SETTING
The architecture of the
current-mode control. The current limit is determined by three
components: the R
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
can vary over temperature and usually has a positive T
that it increases with temperature); therefore, it is recommended to
program the current sense, gain resistor based on the rated R
the MOSFET at 125°C.
Because the
control, the relationship between I
where:
K
desired average load current (see Figure 70).
I
I
Establishing K
Inductor Selection section), but in most cases, K
CLIM
LOAD
I
is the ratio between the inductor ripple current and the
Figure 69. RES Detect Circuit for Current Sense Gain Programming
is the desired valley current limit.
is the current load.
Figure 70. Valley Current Limit to Average Current Relation
ADP1878/ADP1879
I
helps to determine the inductor value (see the
CS GAIN
LOAD CURRENT
AMP
SET
CS
ON
VALLEY CURRENT LIMIT
of the low-side MOSFET, the output voltage
RES
1
ADP1878/ADP1879
ADC
2
RIPPLE CURRENT =
are based on valley current
ADP1878/ADP1879
CLIM
ON
A
3 V/V
6 V/V
12 V/V
24 V/V
0.4V
and I
CS
of the low-side MOSFET
is based on valley
LOAD
SW
PGND
I
is
I
LOAD
= 0.33.
3
C
(meaning
ON
of