Low Input Voltage, Adjustable Frequency Dual Synchronous
2
A / 2
A Buck Regulator with Synchronization, 2× EN, and 2× NPOR
A8651
13
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
FUNCTIONAL DESCRIPTION
Overview
The A8651 is a dual synchronous PWM regulator that incorpo-
rates all the control and protection circuitry necessary to satisfy
a wide range of applications. The A8651 employs current mode
control to provide fast transient response, simple compensa-
tion, and excellent stability. The features of the A8651 include,
for each of the two regulators: a precision reference, an adjust-
able switching frequency, a transconductance error amplifier, an
enable/synchronization input, integrated high-side and low-side
MOSFETs, adjustable Soft-Start, pre-bias startup, low-current
sleep mode, and a Power-On Reset output (NPOR). The protec-
tion features of the A8651 include undervoltage lockout (UVLO),
pulse-by-pulse overcurrent protection (OCP), hiccup mode
short-circuit protection (HIC), overvoltage protection (OVP), and
thermal shutdown (TSD). In addition, the A8651 provides open-
circuit, adjacent pin short-circuit, and pin-to-ground short-circuit
protection.
Reference Voltage
The A8651 incorporates an internal reference that allows output
voltages as low as 0.8 V. The accuracy of the internal reference is
±1% across the operating temperature range. The output voltage
for each of the regulators is adjusted by connecting a resistor
divider (RFB1-RFB2 and RFB3-RFB4 in the Typical Application
diagram) from VOUTx to the corresponding FBx pin of the A8651.
Oscillator/Switching Frequency (RFSET, fOSC)
The PWM switching frequency of the A8651 is adjustable from
350 kHz to 2.2 MHz and has an accuracy of about ±10% across
the operating temperature range. Connecting a resistor ( RFSET
)
from the FSET/SYNC pin to GND, as shown in the Typical
Application diagram, sets the base switching frequency, fOSC
.
An FSET/SYNC resistor with ±1% tolerance is recommended.
A graph of switching frequency versus RFSET resistor value is
shown in the Design and Component Selection section of this
datasheet.
Transconductance Error Amplifier
The transconductance error amplifier’s primary function is to
regulate the converter’s output voltage. The error amplifier for
one of the regulators is shown in Figure 1. It is shown as a three-
terminal input device with two positive and one negative input.
The negative input is simply connected to the FBx pin and is used
to sense the feedback voltage for regulation. The two positive
inputs are used for soft-start and regulation. The error ampli-
fier performs an “analog OR” selection between its two positive
inputs. The error amplifier regulates to either the soft-start pin
voltage minus 200 mV or the A8651’s internal reference, which-
ever is lower.
To stabilize the regulator, a series RC compensation network
(RZ and CZ
) must be connected from the error amplifier output
(COMPx pin) to GND as shown in the Typical Applications
diagram. In some applications, an additional, low-value capacitor
(CP
) may be connected in parallel with the RZ-CZ compensation
network to reduce the loop gain at higher frequencies. However
if the CP capacitor is too large, the phase margin of the converter
may be reduced.
If the regulator is disabled or a fault occurs, the correspond-
ing COMPx pin is immediately pulled to GND via approxi-
mately 1.5 kΩ and PWM switching is inhibited. During startup
(VSSx < 500 mV), the transconductance of the error amplifier is
reduced to approximately one-third of the normal operating level
to minimize transients when the system is requesting on-times
less than or equal to the minimum controllable on-time.
Slope Compensation
The A8651 incorporates internal slope compensation to allow
PWM duty cycles above 50% for a wide range of input/output
voltages, switching frequencies, and inductor values. As shown in
the Detailed Functional Block diagram, the slope compensation
signal is added to the sum of the current sense and PWM Ramp
Offset (VPWMOFFSET
). The amount of slope compensation is
scaled directly with the switching frequency.
Figure 1: The A8651 Error Amplier (for one regulator)
+
-
+
Error Amplifier
COMPx
SSx
FBx
V
REF
800 mV
200 mV