MAX1519 Datasheet, PDF (25/43 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
with input voltage feed forward (Figure 5). This archi-
tecture relies on the output filter capacitor’s ESR to act
as the current-sense resistor, so the output ripple volt-
age provides the PWM ramp signal. The control algo-
rithm is simple: the high-side switch on-time is
determined solely by a one-shot whose period is
inversely proportional to the input voltage, and directly
proportional to the output voltage or the difference
between the main and secondary inductor currents
(see the On-Time One-Shot (TON) section). Another
one-shot sets a minimum off-time. The on-time one-shot
triggers when the error comparator goes low, the induc-
tor current of the selected phase is below the valley current-
limit threshold, and the minimum off-time one-shot times out.
The controller maintains 180° out-of-phase operation by
alternately triggering the main and secondary phases after
the error comparator drops below the output voltage set
point.
On-Time One-Shot (TON)
The core of each phase contains a fast, low-jitter,
adjustable one-shot that sets the high-side MOSFETs
on-time. The one-shot for the main phase varies the on-
time in response to the input and feedback voltages.
The main high-side switch on-time is inversely propor-
tional to the input voltage as measured by the V+ input,
and proportional to the feedback voltage (V_FB):
tON(MAIN) = K(VFB + 0.075V)
V_IN
where K is set by the TON pin-strap connection (Table 6)
and 0.075V is an approximation to accommodate the
expected drop across the low-side MOSFET switch.
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 differ-
ence between the master and slave current-sense sig-
nals. 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
Table 6. Approximate K-Factor Errors
TON
CONNECTION
FREQUENCY
SETTING
(kHz)
K-FACTOR
(µs)
MAX
K-FACTOR
ERROR
(%)
V_CC
Float
REF
GND
100
10
±10
200
5
±10
300
3.3
±10
550
1.8
±12.5
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.
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