MAX1544 Datasheet, PDF (29/42 Pages) Maxim Integrated Products – Dual-Phase, Quick-PWM Controller for AMD Hammer CPU Core Power Supplies

Dual-Phase, Quick-PWM Controller for
AMD Hammer CPU Core Power Supplies
SKIP is a three-level logic input—GND, REF, or high.
This input is intended to be driven by a dedicated
open-drain output with the pullup resistor connected
either to REF (or a resistive divider from V_CC) or to a
logic-level high-bias supply (3.3V or greater).
When driven to GND, the multiphase Quick-PWM con-
troller disables the secondary phase (DLS = PGND and
DHS = LXS) and the primary phase uses the automatic
pulse-skipping control scheme. When pulled up to REF,
the controller keeps both phases active and uses the
automatic pulse-skipping control scheme—alternating
between the primary and secondary phases with each
cycle.
Automatic Pulse-Skipping Switchover
In skip mode (SKIP = REF or GND), an inherent automatic
switchover to PFM takes place at light loads (Figure 7). A
comparator that truncates the low-side switch on-time at
the inductor current’s zero crossing affects this
switchover. The zero-crossing comparator senses the
inductor current across the current-sense resistors. Once
V_C__P - V_C__N drops below the zero crossing comparator
threshold (see the Electrical Characteristics), the com-
parator forces DL low (Figure 5). This mechanism causes
the threshold between pulse-skipping PFM and nonskip-
ping PWM operation to coincide with the boundary
between continuous and discontinuous inductor-current
operation. The PFM/PWM crossover occurs when the
load current of each phase is equal to 1/2 the peak-to-
peak ripple current, which is a function of the inductor
value (Figure 7). For a battery input range of 7V to 20V,
this threshold is relatively constant, with only a minor
dependence on the input voltage due to the typically low
duty cycles. The total load current at the PFM/PWM
crossover threshold (I_LOAD(SKIP)) is approximately:
I_LOAD(SKIP) =
η _TOTAL


V_OUTK
L


 V_IN - V_OUT 
 V_IN 
where η_TOTAL is the number of active phases, and K is
the on-time scale factor (Table 6).
The switching waveforms may appear noisy and asyn-
chronous when light loading activates pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Varying the inductor
value makes trade-offs between PFM noise and light-load
efficiency. Generally, low inductor values produce a
broader efficiency vs. load curve, while higher values
result in higher full-load efficiency (assuming that the coil
resistance remains fixed) and less output voltage ripple.
Penalties for using higher inductor values include larger
physical size and degraded load-transient response,
especially at low input voltage levels.
∆i ₌ V_BATT - V_OUT
∆t
L
I_PEAK
I_LOAD = I_PEAK/2
0 ON-TIME
TIME
Figure 7. Pulse-Skipping/Discontinuous Crossover Point
I_PEAK
I_LOAD
I_LIMIT
( ) ^{ILIMIT(VALLEY) = ILOAD(MAX)}
2 - LIR
2η
0
TIME
Figure 8. “Valley” Current-Limit Threshold Point
Current-Limit Circuit
The current-limit circuit employs a unique “valley” cur-
rent-sensing algorithm that uses current-sense resistors
between the current-sense inputs (C_P to C_N) as the
current-sensing elements. If the current-sense signal of
the selected phase is above the current-limit threshold,
the PWM controller does not initiate a new cycle
(Figure 8) until the inductor current of the selected
phase drops below the valley current-limit threshold.
When either phase trips the current limit, both phases
are effectively current limited since the interleaved con-
troller does not initiate a cycle with either phase.
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