Detailed description – Rainbow Electronics MAX17409 User Manual
Page 16
MAX17409
1-Phase Quick-PWM GPU Controller
16
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Detailed Description
Free-Running, Constant On-Time PWM
Controller with Input Feed-Forward
The Quick-PWM control architecture is a pseudo-fixed-
frequency, constant-on-time, current-mode regulator
with voltage feed-forward (Figure 2). This architecture
relies on the output filter capacitor’s ESR to act as the
current-sense resistor, so the output ripple voltage pro-
vides the PWM ramp signal. The control algorithm is
simple: the high-side switch on-time is determined solely
by a one-shot whose period is inversely proportional to
input voltage, and directly proportional to output volt-
age (see the
On-Time One-Shot
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 is below the valley current-limit threshold,
and the minimum off-time one-shot times out.
+5V Bias Supply (V
CC
and V
DD
)
The Quick-PWM controller requires an external +5V
bias supply in addition to the battery. Typically, this
+5V bias supply is the notebook’s 95% efficient +5V
system supply. Keeping the bias supply external to the
IC improves efficiency and eliminates the cost associat-
ed with the +5V linear regulator that would otherwise be
needed to supply the PWM circuit and gate drivers. If
stand-alone capability is needed, the +5V bias supply
can be generated with an external linear regulator.
The +5V bias supply must provide V
CC
(PWM con-
troller) and V
DD
(gate-drive power), so the maximum
current drawn is:
where I
CC
is provided in the
Electrical Characteristics
table, f
SW
is the switching frequency, and Q
G(LOW)
and
Q
G(HIGH)
are the MOSFET data sheet’s total gate-
charge specification limits at V
GS
= 5V.
V
IN
and V
DD
can be connected together if the input
power source is a fixed +4.5V to +5.5V supply. If the
+5V bias supply is powered up prior to the battery sup-
ply, the enable signal (SHDN going from low to high)
must be delayed until the battery voltage is present to
ensure startup.
Switching Frequency (TON)
Connect a resistor (R
TON
) between TON and V
IN
to set
the switching period (t
SW
= 1/f
SW
):
t
SW
= 16.3pF x (R
TON
+ 6.5k
Ω)
A 96.75k
Ω to 303.25kΩ corresponds to switching peri-
ods of 167ns (600kHz) to 500ns (200kHz), respectively.
High-frequency (600kHz) operation optimizes the appli-
cation for the smallest component size, trading off effi-
ciency due to higher switching losses. This might be
acceptable in ultra-portable devices where the load
currents are lower and the controller is powered from a
lower voltage supply. Low-frequency (200kHz) opera-
tion offers the best overall efficiency at the expense of
component size and board space.
On-Time One-Shot
The core contains a fast, low-jitter, adjustable one-shot
that sets the high-side MOSFET’s on-time. The one-shot
varies the on-time in response to the input and feed-
back voltages. The main high-side switch on-time is
inversely proportional to the input voltage as measured
by the R
TON
input, and proportional to the feedback
voltage (V
FB
):
where the switching period (t
SW
= 1/f
SW
) is set by the
resistor at the TON pin and 0.075V is an approximation
to accommodate the expected drop across the low-
side MOSFET switch.
This algorithm results in a nearly constant switching fre-
quency and balanced inductor currents despite the lack
of a fixed-frequency clock generator. The benefits of a
constant switching frequency are twofold: first, the fre-
quency 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
table. On-
times at operating points far removed from the condi-
tions specified in the
Electrical Characteristics
table
can vary over a wider range.
On-times translate only roughly to switching frequen-
cies. The on-times guaranteed in the
Electrical
Characteristics
table are influenced by switching
delays in the external high-side MOSFET. Resistive
losses, including the inductor, both MOSFETs, output
capacitor ESR, and PCB 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, reducing the switching
frequency. It occurs only during forced-PWM operation
and dynamic output-voltage transitions when the induc-
tor current reverses at light or negative load currents.
With reversed inductor current, the inductor’s EMF
causes LX to go high earlier than normal, extending the
t
t
V
V
V
ON MAIN
SW
FB
IN
(
)
.
=
+
(
)
0 075
I
I
f
Q
Q
BIAS
CC
SW
G LOW
G HIGH
=
+
+
(
)
(
)
(
)