MAX1519 Datasheet, PDF (29/43 Pages) Maxim Integrated Products – Dual-Phase, Quick-PWM Controllers for Programmable CPU Core Power Supplies

Dual-Phase, Quick-PWM Controllers for
Programmable CPU Core Power Supplies
200
UNDEFINED
REGION
100
∆i ₌ V_BATT - V_OUT
∆t
L
I_PEAK
0
I_LOAD = I_PEAK/2
-100
-200
0
0.8 1.2
0.5
1.0
1.5
2.0
OFS VOLTAGE (V)
0 ON-TIME
TIME
Figure 6. Offset Voltage
Figure 7. Pulse-Skipping/Discontinuous Crossover Point
reverse under light loads, providing fast, accurate neg-
ative output voltage transitions by quickly discharging
the output capacitors.
Forced-PWM operation comes at a cost: the no-load 5V
bias supply current remains between 10mA to 60mA
per phase, depending on the external MOSFETs and
switching frequency. To maintain high efficiency under
light load conditions, the processor may switch the
controller to a low-power pulse-skipping control
scheme after entering suspend mode.
Low-Power Pulse Skipping
During pulse-skipping override mode (SKIP = REF or
GND, Table 7), the multiphase Quick-PWM controllers
use an automatic pulse-skipping control scheme. When
SKIP is pulled low, the controller uses the automatic
pulse-skipping control scheme, overriding forced-PWM
operation, and blanks the upper VROK threshold.
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 auto-
matic switchover to PFM takes place at light loads
(Figure 7). A comparator that truncates the low-side
I_PEAK
I_LOAD
I_LIMIT
( ) ^{ILIMIT(VALLEY) = ILOAD(MAX)}
2 - LIR
2η
0
TIME
Figure 8. “Valley” Current-Limit Threshold Point
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 comparator forces DL low (Figure 5).
This mechanism causes the threshold between pulse-
skipping PFM and nonskipping PWM operation to coin-
cide 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 cur-
rent, 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
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