Rainbow Electronics MAX1545 - Manual (Page 25)

MAX1519/MAX1545
Dual-Phase, Quick-PWM Controllers for
Programmable CPU Core Power Supplies
______________________________________________________________________________________ 25
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 ):
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:
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:
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.
t K
V V
V
K
V V
V
K
I Z
V
Main On Time
Secondary Current Balance Correction
ON ND
CCI
IN
FB
IN
CCI CCI
IN
( )
.
.
( )
( )
2
0 075
0 075
=
+ 
 

=
+ 
 

+

 

= +
−
I G V V G V V
V V I Z
CCI M CMP CMN M CSP CSN
CCI FB CCI CCI
=
( ) ( )
=
+
- - -
t
ON MAIN
K V V
V
FB
IN
( )
.
=
+
( )
0 075
Table 6. Approximate K-Factor Errors
TON
CONNECTION
FREQUENCY
SETTING
(kHz)
K-FACTOR
(µs)
MAX
K-FACTOR
ERROR
(%)
V CC 100 10 ±10
Float 200 5 ±10
REF 300 3.3 ±10
GND 550 1.8 ±12.5