ADP1876-EVALZ AD [Analog Devices], ADP1876-EVALZ Datasheet - Page 14

• Manufacturer Part Number: ADP1876-EVALZ
• Description: 600 kHz Dual Output Synchronous Buck
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
The
controller with integrated drivers that drive N-channel power
MOSFETs. The device operates in current mode for improved
transient response and uses valley current sensing for enhanced
noise immunity. The two outputs are phase shifted 180°. This
reduces the input current ripple and the required input
capacitance.
The integrated boost diodes in the
system cost and component count. The
fixed frequency of 600 kHz and includes programmable soft
start, current limit, and power good.
INDEPENDENT LOW DROPOUT LINEAR
REGULATOR
In addition to the dual channel step-down controller, a stand-
alone linear dropout (LDO) voltage regulator with a fixed output
of 1.5 V is built into the
from the controllers. The output of the LDO delivers up to
150 mA to the load. See the Applications Information section
for more information.
CONTROLLER ARCHITECTURE
The
PWM control architecture. The inductor current is sensed by
the voltage drop measured across the external low-side MOSFET
R
current). The current sense signal is further processed by the
current sense amplifier. The output of the current sense amplifier is
held, and the emulated current ramp is multiplexed and fed into
the PWM comparator (see Figure 24). The valley current
information is captured at the end of the off period, and the
emulated current ramp is applied at that point when the next on
cycle begins. An error amplifier integrates the error between the
feedback voltage and the generated error voltage from the
COMP pin (see the “from error amp” in Figure 24).
I
ADP1876
RAMP
DSON
ADP1876
ADP1876
during the off period of the switching cycle (valley inductor
V
A
IN
R
R
RAMP
is a dual output dc-to-dc synchronous buck
is based on a fixed frequency, current mode
Figure 24. Simplified Control Architecture
V
IN
C
R
ADP1876
FROM
ERROR AMP
V
and operates independently
CS
ADP1876
OSC
ADP1876
A
CS
S
R
reduce the overall
FF
Q
Q
FROM
LOW SIDE
MOSFET
operates at a
TO
DRIVERS
Rev. A | Page 14 of 24
As shown in Figure 24, the emulated current ramp is generated
inside the IC but offers programmability through the RAMPx
pin (see Figure 1 for the typical operation circuit). Selecting an
appropriate value resistor from V
a desired slope compensation value and, at the same time,
provides a feedforward feature. The benefits realized by
deploying this type of control scheme are as follows:
• The turn-on current spike does not corrupt the current ramp.
• The current signal is stable because the current signal is
The normal benefits of using current mode control scheme still
apply, such as simplicity of loop compensation. Control logic
enforces antishoot through operation to limit cross conduction of
the internal drivers and external MOSFETs.
Synchronous Rectifier and Dead Time
The synchronous rectifier (low-side MOSFET) improves efficiency
by replacing the Schottky diode that is normally used in an
asynchronous buck regulator. In the ADP1876, the antishoot
through circuit monitors the SW and DL nodes and adjusts the
low-side and high-side drivers to ensure break-before-make
switching to prevent cross conduction or shoot through between
the high-side and low-side MOSFETs. This break-before-make
switching is known as the dead time, which is not fixed and
depends on how fast the MOSFETs are turned on and off. In a
typical application circuit that uses medium sized MOSFETs
with input capacitance of approximately 3 nF, the typical dead
time is approximately 30 ns. When small and fast MOSFETs are
used, the dead time can be as low as 13 ns.
INPUT UNDERVOLTAGE LOCKOUT
When the bias input voltage, V
lockout (UVLO) threshold, the switch drivers stay inactive.
When V
switching.
INTERNAL LINEAR REGULATOR (VCCO)
The internal linear regulator is low dropout, meaning it can
regulate its output voltage, VCCO. VCCO powers the internal
control circuitry and provides power for the gate drivers. It is guar-
anteed to have more than 200 mA of output current capability,
which is sufficient to handle the gate drive requirements of
typical logic threshold MOSFETs. VCCO is always active and
cannot be shut down by the EN1/EN2 pins. Bypass VCCO to
AGND with a 1 µF or greater capacitor.
Because the LDO supplies the gate drive current, the output of
VCCO is subject to sharp transient currents as the drivers switch
and the boost capacitors recharge during each switching cycle.
sampled at the end of the turn-off period, which gives time
for the switch node ringing to settle.
IN
exceeds the UVLO threshold, the switchers begin
IN
, is less than the undervoltage
IN
to the RAMPx pin programs
Data Sheet