MAX1544 Datasheet, PDF (36/42 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
MAX1544
ERROR
COMPARATOR
CMN
CMP
MAIN
PHASE
OAIN+
OAIN-
FB
R_F
SECOND
PHASE
CSP
CSN
L1
R_SENSE
R_A R_B
PC BOARD TRACE
RESISTANCE
R_FBS
CPU SENSE
POINT
R_A R_B
L2
R_SENSE
PC BOARD TRACE
RESISTANCE
Figure 10. Voltage-Positioning Gain
slew the inductor current higher in response to
increased load, and must always be greater than 1. As
h approaches 1, the absolute minimum dropout point,
the inductor current cannot increase as much during
each switching cycle and V_SAG greatly increases
unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but adjusting
this up or down allows tradeoffs between V_SAG, output
capacitance, and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as:


V_IN(MIN)
=
η _OUTPH


¹
-
V_FB - V_VPS
η_OUTPH ^{ h}
+
x
V_DROP1
t_OFF(MIN)
K





+ V_DROP2 - V_DROP1 + V_VPS
where η_OUTPH is the total number of out-of-phase switch-
ing regulators, V_VPS is the voltage-positioning droop,
V_DROP1 and V_DROP2 are the parasitic voltage drops in
the discharge and charge paths (see the On-Time One-
Shot (TON) section), t_OFF(MIN) is from the Electrical
Characteristics, and K is taken from Table 6. The
absolute minimum input voltage is calculated with h = 1.
If the calculated V_IN(MIN) is greater than the required min-
imum input voltage, then reduce the operating frequency
or add output capacitance to obtain an acceptable V_SAG.
If operation near dropout is anticipated, calculate V_SAG to
be sure of adequate transient response.
Dropout design example:
V_FB = 1.4V
K_MIN = 3µs for f_SW = 300kHz
t_OFF(MIN) = 400ns
V_VPS = 3mV/A × 30A = 90mV
V_DROP1 = V_DROP2 = 150mV (30A load)
h = 1.5 and η_OUTPH = 2
V_IN(MIN)
=
2
x

¹
1.4V
-2x
- 90mV + 150mV 
(0.4µs
x
1.5
/
3.0µs


+ 150mV - 150mV + 90mV = 4.96V
Calculating again with h = 1 gives the absolute limit of
dropout:
V_IN(MIN)
=
2
x

¹
1.4V
-2x
- 90mV + 150mV 
(0.4µs
x
1.0
/
3.0µs


+ 150mV - 150mV + 90mV = 4.07V
Therefore, V_IN must be greater than 4.1V, even with very
large output capacitance, and a practical input voltage
with reasonable output capacitance would be 5V.
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