ADP1882 Analog Devices, ADP1882 Datasheet - Page 19

• Manufacturer Part Number: ADP1882
• Description: Synchronous Current-Mode Buck Controller with Constant On-time and 0.8 V Reference Voltage
• Manufacturer: Analog Devices

THEORY OF OPERATION
The ADP1882/ADP1883 are versatile current-mode, synchronous
step-down controllers that provide superior transient response,
optimal stability, and current limit protection by using a constant
on-time, pseudo-fixed frequency with a programmable current-
sense gain, current-control scheme. In addition, these devices offer
optimum performance at low duty cycles by using valley current-
mode control architecture. This allows the ADP1882/ADP1883
to drive all N-channel power stages to regulate output voltages
as low as 0.8 V.
STARTUP
The ADP1882/ADP1883 have an input low voltage pin (VDD) for
biasing and supplying power for the integrated MOSFET drivers. A
bypass capacitor should be located directly across the VDD (Pin 5)
and PGND (Pin 7) pins. Included in the power-up sequence is
the biasing of the current-sense amplifier, the current-sense gain
circuit (see the Programming Resistor (RES) Detect Circuit
section), the soft start circuit, and the error amplifier.
The current-sense blocks provide valley current information
(see the Programming Resistor (RES) Detect Circuit section)
and are a variable of the compensation equation for loop stability
(see the Compensation Network section). The valley current
information is extracted by forcing 0.4 V across the DRVL output
and the PGND pin, which generates a current depending on the
resistor across DRVL and PGND in a process performed by the
RES detect circuit. The current through the resistor is used to set
the current-sense amplifier gain. This process takes approximately
800 μs, after which the drive signal pulses appear at the DRVL
and DRVH pins synchronously and the output voltage begins to
rise in a controlled manner through the soft start sequence.
The rise time of the output voltage is determined by the soft
start and error amplifier blocks (see the Soft Start section).
At the beginning of a soft start, the error amplifier charges the
external compensation capacitor, causing the COMP/EN pin to
rise above the enable threshold of 285 mV, thus enabling the
ADP1882/ADP1883.
SOFT START
The ADP1882/ADP1883 have digital soft start circuitry, which
involves a counter that initiates an incremental increase in current,
by 1 μA, via a current source on every cycle through a fixed internal
capacitor. The output tracks the ramping voltage by producing
PWM output pulses to the upper-side MOSFET. The purpose is to
limit the in-rush current from the high voltage input supply (VIN)
to the output (V
PRECISION ENABLE CIRCUITRY
The ADP1882/ADP1883 employ precision enable circuitry. The
enable threshold is 285 mV typical with 35 mV of hysteresis.
The devices are enabled when the COMP/EN pin is released,
allowing the error amplifier output to rise above the enable
threshold (see Figure 66). Grounding this pin disables the
OUT
).
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ADP1882/ADP1883, reducing the supply current of the devices
to approximately 140 μA. For more information, see Figure 67.
UNDERVOLTAGE LOCKOUT
The undervoltage lockout (UVLO) feature prevents the part
from operating both the upper-side and lower-side MOSFETs
at extremely low or undefined input voltage (V
Operation at an undefined bias voltage may result in the
incorrect propagation of signals to the high-side power
switches. This, in turn, results in invalid output behavior that
can cause damage to the output devices, ultimately destroying
the device tied at the output. The UVLO level has been set at
2.65 V (nominal).
THERMAL SHUTDOWN
The thermal shutdown is a self-protection feature to prevent the
IC from damage due to a very high operating junction temperature.
If the junction temperature of the device exceeds 155°C, the part
enters the thermal shutdown state. In this state, the device shuts off
both the upper-side and lower-side MOSFETs and disables the
entire controller immediately, thus reducing the power consump-
tion of the IC. The part resumes operation after the junction
temperature of the part cools to less than 140°C.
Figure 66. Release COMP/EN Pin to Enable the ADP1882/ADP1883
500mV
285mV
>2.4V
2.4V
0.9V
COMP/EN
0V
R
C
C
COMP/EN
C
Figure 67. COMP/EN Voltage Range
C
C2
FB
HICCUP MODE INITIALIZED
MAXIMUM CURRENT (UPPER CLAMP)
PRECISION ENABLE THRESHOLD
ZERO CURRENT
LOWER CLAMP
35mV HYSTERESIS
USABLE RANGE ONLY AFTER SOFT START
PERIOD IF CONTUNUOUS CONDUCTION
MODE OF OPERATION IS SELECTED.
ADP1882/ADP1883
ERROR
AMPLIFIER
250mV
VDD
PRECISION
ENABLE
ADP1882/ADP1883
TO ENABLE
ALL BLOCKS
SS
0.8V
DD
) ranges.