MAX1937/MAX1938/MAX1939
Two-Phase Desktop CPU Core Supply
Controllers with Controlled VID Change
14 ______________________________________________________________________________________
EMF causes LX to go high earlier than normal, extend-
ing the on-time by a period equal to the DH rising
dead-time.
When the controller operates in continuous mode, the
dead-time is no longer a factor, and the actual switch-
ing frequency is:
where V
DROP1
is the sum of the parasitic voltage drops
in the inductor discharge path, including the synchro-
nous rectifier, inductor, and PC board resistances;
V
DROP2
is the sum of the resistances in the charging
path, including the high-side MOSFET, inductor, and
PC board resistances.
Synchronized 2-Phase Operation
The two phases of the MAX1937/MAX1938/MAX1939
operate 180° out-of-phase to reduce input filtering
requirements, reduce electromagnetic interference
(EMI), and improve efficiency. This effectively lowers
cost and saves board space, making the MAX1937/
MAX1938/MAX1939 ideal for cost-sensitive applica-
tions.
With dual synchronized out-of-phase operation, the
MAX1937/MAX1938/MAX1939s’ high-side MOSFETs turn
on 180° out-of-phase. The instantaneous input current
peaks of both regulators do not overlap, resulting in
reduced input voltage ripple and RMS ripple current.
This reduces the input capacitance requirement, allowing
fewer or less expensive capacitors, and reduces shield-
ing requirements for EMI. The 180° out-of-phase wave-
forms are shown in the Typical Operating Characteristics.
Each phase operates with a 250kHz switching frequen-
cy. Since the two regulators operate 180° out-of-phase,
an effective switching of 500kHz is seen at the input
and output. In addition to being at a higher frequency
(compared to a single-phase regulator), both the input
and output ripple have lower amplitude.
Phase Overlap
To minimize the crosstalk noise in the two phases, the
maximum duty cycle of the MAX1937/MAX1938/
MAX1939 is less than 50%. To provide a fast transient
response, these devices have a phase-overlap mode
that allows the two phases to operate in phase when a
heavy-load transient is detected. In-phase operation
continues until the output voltage returns to the nominal
output voltage regulation value.
Once regulation is achieved, the controller returns to
180° out-of-phase operation. A minimum current-adap-
tive phase-selection algorithm is used to determine which
phase is used to start the first out-of-phase cycle. Once
the output voltage returns to the nominal output voltage
regulation value, the subsequent cycle starts with the
phase that has the lowest inductor current. For example,
if the current-sense inputs indicate that phase 2 has
lower inductor current than phase 1, the controller turns
on phase 2’s high-side MOSFET first when returning to
normal operation.
Differential Voltage Sensing and Error
Comparator
The MAX1937/MAX1938/MAX1939 use differential
sensing of the output voltage to achieve the highest
possible accuracy of the output voltage. This allows the
error comparator to sense the actual voltage at the
load, so that the controller can compensate for losses
in the power output and ground lines.
FB and GNDS are used for the differential output voltage
sensing. The controller triggers the next cycle (turn on
the high-side MOSFET) when the error comparator is low
(V
FB
- V
GNDS
is less than the VID regulation voltage),
V
CS
is below the current-limit threshold, and the mini-
mum off-time one-shot has timed out.
Traces from FB and GNDS should be routed close to
each other and as far away as possible from sources of
noise (such as the inductors and high di/dt traces). If
noise on these connections cannot be prevented, then
use RC filters. To filter FB, connect a 100Ω series resistor
from the positive sense trace to FB, and connect a
1000pF capacitor from FB to GND right at the FB pin. For
GNDS, connect a 100Ω series resistor from the negative
sense trace to GNDS, and connect a 1000pF capacitor
from GNDS to GND at the GNDS pin.
For VRM applications, connect a 10kΩ resistor from FB
to the output locally (on the VRM board), and connect a
10kΩ resistor from GNDS to PGND locally (on the VRM
board). FB and GNDS also connect to the output at the
load (off the VRM board, at the microprocessor). This
provides the benefits of differential output voltage sens-
ing mentioned above and the safety of regulating the
output voltage on the board in case the external sense
connections get disconnected.
External Linear Regulator
A 6V linear regulator (U2) is used to step down the
main supply. The output of this linear regulator is con-
nected to VLG to provide power for the low-side gate
drive and bootstrap circuit. Using 6V for this supply
improves efficiency by providing a stronger gate drive
than a 5V supply. To reduce switching noise on VLG,
