Rainbow Electronics MAX8760 - Manual (Page 16)

MAX8760
Dual-Phase, Quick-PWM Controller for AMD
Mobile Turion 64 CPU Core Power Supplies
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Detailed Description
Dual 180° Out-of-Phase Operation
The two phases in the MAX8760 operate 180° out-of-
phase (SKIP = REF or high) to minimize input and output
filtering requirements, reduce electromagnetic interfer-
ence (EMI), and improve efficiency. This effectively low-
ers component count—reducing cost, board space, and
component power requirements—making the MAX8760
ideal for high-power, cost-sensitive applications.
Typically, switching regulators provide transfer power
using only one phase instead of dividing the power
among several phases. In these applications, the input
capacitors must support high-instantaneous current
requirements. The high-RMS ripple current can lower
efficiency due to I
2 R power loss associated with the input
capacitor’s effective series resistance (ESR). Therefore,
the system typically requires several low-ESR input
capacitors in parallel to minimize input voltage ripple, to
reduce ESR-related power losses, and to meet the nec-
essary RMS ripple current rating.
With the MAX8760, the controller shares the current
between two phases that operate 180° out-of-phase, so
the high-side MOSFETs never turn on simultaneously
during normal operation. The instantaneous input cur-
rent of either phase is effectively cut in half, resulting in
reduced input voltage ripple, ESR power loss, and RMS
ripple current (see the Input Capacitor Selection sec-
tion). As a result, the same performance can be
achieved with fewer or less-expensive input capacitors.
Table 1 lists component selection for standard multi-
phase selections and Table 2 is a list of component
suppliers.
Transient Overlap Operation
When a transient occurs, the response time of the con-
troller depends on how quickly it can slew the inductor
current. Multiphase controllers that remain 180 degrees
out-of-phase when a transient occurs actually respond
slower than an equivalent single-phase controller. To
provide fast transient response, the MAX8760 supports
a phase-overlap mode, which allows the dual regula-
tors to operate in-phase when heavy-load transients are
detected, reducing the response time. After either high-
side MOSFET turns off, if the output voltage does not
exceed the regulation voltage when the minimum off-
time expires, the controller simultaneously turns on both
high-side MOSFETs during the next on-time cycle. This
maximizes the total inductor current slew rate. The
phases remain overlapped until the output voltage
exceeds the regulation voltage after the minimum off-
time expires.
After the phase-overlap mode ends, the controller auto-
matically begins with the opposite phase. For example,
if the secondary phase provided the last on-time pulse
before overlap operation began, the controller starts
switching with the main phase when overlap operation
ends.
Power-Up Sequence
The MAX8760 is enabled when SHDN is driven high
(Figure 2). The reference powers up first. Once the ref-
erence exceeds its UVLO threshold, the PWM controller
evaluates the DAC target and starts switching.
Pin Description (continued)
PIN NAME FUNCTION
32 DLS
S econd ar y Low - S i d e G ate- D r i ver O utp ut. D LS sw i ng s fr om P GN D to V D D . D LS i s for ced hi g h after the
M AX 8760 p ow er s d ow n.
33 DHS Secondary High-Side Gate-Driver Output. Swings LXS to BSTS.
34 LXS Secondary Inductor Connection. LXS is the internal lower supply rail for the DHS high-side gate driver.
35 BSTS
Secondary Boost Flying Capacitor Connection. An optional resistor in series with BSTS allows the DHS
pullup current to be adjusted.
36 V+
Battery Voltage-Sense Connection. Used only for PWM one-shot timing. DH_ on-time is inversely
proportional to input voltage over a 4V to 28V range.
37 CMP Main Inductor Positive Current-Sense Input
38 CMN Main Inductor Negative Current-Sense Input
39 CSN Secondary Inductor Positive Current-Sense Input
40 CSP Secondary Inductor Negative Current-Sense Input