MAX1544 Datasheet, PDF (26/42 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
The one-shot for the secondary phase varies the on-time
in response to the input voltage and the difference
between the main and secondary inductor currents.
Two identical transconductance amplifiers integrate the
difference between the master and slave current-sense
signals. The summed output is internally connected to
CCI, allowing adjustment of the integration time constant
with a compensation network connected between CCI
and FB.
The resulting compensation current and voltage are
determined by the following equations:
( ) ( ) ^I_CCI ^{= G}_M ^V_CMP ^{- V}_CMN ^{- G}_M ^V_CSP ^{- V}_CSN
V_CCI = V_{FB +} I_CCIZ_CCI
where Z_CCI is the impedance at the CCI output. The
secondary on-time one-shot uses this integrated signal
(V_CCI) to set the secondary high-side MOSFETs on-time.
When the main and secondary current-sense signals
(V_CM = V_CMP - V_CMN and V_CS = V_CSP - V_CSM) become
unbalanced, the transconductance amplifiers adjust the
secondary on-time, which increases or decreases the
secondary inductor current until the current-sense
signals are properly balanced:
t_ON(2ND) =
K


V_CCI
+ 0.075V 
V_IN

=
K


V_FB
+ 0.075V 
V_IN

+

K_
I_CCIZ_CCI
V_IN


= (Main on − time) + (Secondary Current
Balance Correction)
This algorithm results in a nearly constant switching
frequency and balanced inductor currents, despite the
lack of a fixed-frequency clock generator. The benefits of
a constant switching frequency are twofold: first, the
frequency can be selected to avoid noise-sensitive
regions such as the 455kHz IF band; second, the induc-
tor ripple-current operating point remains relatively con-
stant, resulting in easy design methodology and
predictable output-voltage ripple. The on-time one-shots
have good accuracy at the operating points specified in
the Electrical Characteristics. On-times at operating
points far removed from the conditions specified in the
Electrical Characteristics can vary over a wider range. For
example, the 300kHz setting typically runs about 3%
slower with inputs much greater than 12V due to the very
short on-times required.
On-times translate only roughly to switching frequencies.
The on-times guaranteed in the Electrical Characteristics
are influenced by switching delays in the external high-
side MOSFET. Resistive losses, including the inductor,
both MOSFETs, output capacitor ESR, and PC board
copper losses in the output and ground tend to raise the
switching frequency at higher output currents. Also, the
dead-time effect increases the effective on-time, reduc-
ing the switching frequency. It occurs only during forced-
PWM operation and dynamic output voltage transitions
when the inductor current reverses at light or negative
load currents. With reversed inductor current, the induc-
tor’s EMF causes LX to go high earlier than normal,
extending the on-time by a period equal to the DH-rising
dead time.
For loads above the critical conduction point, where the
dead-time effect is no longer a factor, the actual
switching frequency (per phase) is:
( ) _{fSW =}
V_OUT + V_DROP1
( ) t_ON V_IN + V_DROP1 - V_DROP2
where V_DROP1 is the sum of the parasitic voltage drops in
the inductor discharge path, including synchronous recti-
fier, inductor, and PC board resistances; V_DROP2 is the
sum of the parasitic voltage drops in the inductor charge
path, including high-side switch, inductor, and PC board
resistances; and t_ON is the on-time as determined above.
Current Balance
Without active current-balance circuitry, the current
matching between phases depends on the MOSFET’s
on-resistance (R_DS(ON)), thermal ballasting, on-/off-time
matching, and inductance matching. For example, vari-
ation in the low-side MOSFET on-resistance (ignoring
thermal effects) results in a current mismatch that is
proportional to the on-resistance difference:
I_MAIN -I_2ND =
I_MAIN

1-



R_MAIN
R_2ND





However, mismatches between on-times, off-times, and
inductor values increase the worst-case current imbal-
ance, making it impossible to passively guarantee
accurate current balancing.
Table 6. Approximate K-Factor Errors
FREQUENCY
TON
CONNECTION
SETTING
(kHz)
K-FACTOR
(µs)
V_CC
Float
REF
GND
100
10
200
5
300
3.3
550
1.8
MAX
K-FACTOR
ERROR
(%)
±10
±10
±10
±12.5
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