Rainbow Electronics MAX8707 User Manual
Page 17
Switching Frequency (OSC)
OSC is a 3-level logic input used to set the per-phase
switching frequency. Connect OSC directly to GND,
REF, or V
CC
for 200kHz, 300kHz, and 600kHz opera-
tion, respectively. High-frequency (600kHz, OSC =
V
CC
) operation optimizes the application for the small-
est component size, trading off efficiency due to higher
switching losses. This may be acceptable in ultra-
portable devices where the load currents are lower.
Low-frequency (200kHz, OSC = GND) operation offers
the best overall efficiency at the expense of component
size and board space.
Interleaved Multiphase Operation
The MAX8707 interleaves all the active phases—result-
ing in out-of-phase operation that minimizes the input
and output filtering requirements, reduces electromag-
netic interference (EMI), and improves efficiency. The
multiphase controller shares the current between multiple
phases that operate 90
° out-of-phase (4-phase opera-
tion) or 120
° out-of-phase (3-phase operation). The high-
side MOSFETs do not turn on simultaneously during
normal operation. The instantaneous input current is
effectively reduced by the number of active phases,
resulting in reduced input voltage ripple, ESR power loss,
and RMS ripple current (see the Input-Capacitor
Selection section). Therefore, the controller achieves high
performance while minimizing the component count—
which reduces cost, saves board space, and lowers
component power requirements—making the MAX8707
ideal for high-power, cost-sensitive applications.
Transient Phase Repeat
When a transient occurs, the response time of the con-
troller depends on its ability to quickly respond to the
output-voltage deviation and slew the inductor current
to the new current level. Multiphase, fixed-frequency
controllers typically respond only to the clock edge,
resulting in a delayed response from the actual tran-
sient event. To eliminate this delay time, the MAX8707
includes transient phase repeat, which allows the con-
troller to immediately respond when heavy load tran-
sients are detected. If the controller detects that the
output voltage has dropped by 25mV, the transient
detection comparator immediately retriggers the phase
that completed its on-time last. The controller triggers
the subsequent phases as normal—on the appropriate
oscillator edges. This effectively triggers a phase a full
cycle early, increasing the total inductor-current slew
rate and providing an immediate transient response.
Feedback-Adjustment Amplifiers
Voltage-Positioning Amplifier (Steady-State Droop)
The multiphase controllers include a transconductance
amplifier for adding gain to the voltage-positioning
sense path. The current-sense inputs differentially
sense the voltage across either a single current-sense
resistor (CRS sensing enabled) or the inductor’s DCR
(CRS sensing disabled). The VPS amplifier’s input is
generated by sensing either a single phase (CRS sens-
ing) and multiplying by the number of active phases, or
by summing the current-sense (CS_) inputs of all active
phases (CRSP = V
CC
). The transconductance amplifi-
er’s output connects to the regulator’s voltage-posi-
tioned feedback input (VPS), so the resistance between
VPS and the output voltage-sense point (FBS) deter-
mines the voltage-positioning gain:
V
OUT
= V
TARGET
- R
VPS
I
VPS
where the target voltage (V
TARGET
) is defined in the
Nominal Output-Voltage Selection section, and the
transconductance amplifier’s output current (I
VPS
) is
determined by the current-sense voltage and the num-
ber of active phases (
η
PH
):
I
VPS
=
η
PH
(V
CRSP
- V
CRSN
) G
M(VPS)
when CRS sensing is enabled, or:
I
VPS
=
∑ (V
CSP_
- V
CSN_
) G
M(VPS)
when CRS sensing is disabled (CRSP = V
CC
).
where G
M(VPS)
is typically 200µS as defined in the
Electrical Characteristics
table
. To avoid output-voltage
errors caused by the VPS current flowing through para-
sitic trace resistance or feedback fliter resistance, a
second transconductance amplifier generates an equal
and opposite current on the FBS input.
Disable voltage positioning by shorting VPS directly to FBS.
MAX8707
Multiphase, Fixed-Frequency Controller for
AMD Hammer CPU Core Power Supplies
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