Rainbow Electronics MAX17031 - Manual (Page 16)

MAX17031
The MAX17031 includes several features for multipur-
pose notebook functionality, and is specifically
designed for 5V/3.3V main power-supply rails. The
MAX17031 includes a 100mA, 5V linear regulator
(LDO5) ideal for initial power-up of the notebook and
main supply. Additionally, the MAX17031 includes a
3.3V, 5mA RTC supply that remains always enabled,
which can be used to power the RTC supply and sys-
tem pullups when the notebook shuts down. The
MAX17031 also includes a SKIP mode control input
with an accurate threshold that allows an unregulated
charge pump or secondary winding to be automatically
refreshed—ideal for generating the low-power 12V to
15V load switch supply.
3.3V RTC Power
The MAX17031 includes a low-current (5mA) linear reg-
ulator that remains active as long as the input supply
(IN) exceeds 2V (typ). The main purpose of this
“always-enabled” linear regulator is to power the RTC
when all other notebook regulators are disabled. The
RTC regulator sources at least 5mA for external loads.
Preset 5V, 100mA Linear Regulator
The MAX17031 includes a high-current (100mA) 5V lin-
ear regulator. This LDO5 is required to generate the 5V
bias supply necessary to power up the switching regula-
tors. Once the 5V switching regulator (MAX17031 OUT1)
is enabled, LDO5 is bypassed to OUT1. The MAX17031
LDO5 sources at least 100mA of supply current.
Bypass Switch
The MAX17031 includes an LDO5 bypass switch that
allows the LDO5 to be bypassed to OUT1. When OUT1
exceeds 93.5% of the LDO5 output voltage for 500µs,
then the MAX17031 reduces the LDO5 regulation
threshold and turns on an internal p-channel MOSFET to
short OUT1 to LDO5. Instead of disabling the LDO5
when the MAX17031 enables the bypass switch, the
controller reduces the LDO5 regulation voltage, which
effectively places the linear regulator in a standby state
while switched over, allowing a fast recovery if the OUT1
drops by 8.5% from LDO5 nominal regulation threshold.
5V Bias Supply (V CC /V DD )
The MAX17031 requires an external 5V bias supply
(V
DD and V CC ) in addition to the battery. Typically, this
5V bias supply is generated by the internal 100mA
LDO5 or from the notebook’s 95%-efficient 5V main
supply. Keeping these bias supply inputs independent
improves the overall efficiency. When ONLDO is
enabled, V
DD and V CC must be supplied from LDO5.
The V DD bias supply input powers the internal gate dri-
vers and the V CC bias supply input powers the analog
control blocks. The maximum current required is domi-
nated by the switching losses of the drivers and can be
estimated as follows:
I BIAS(MAX) = I CC(MAX) + f SW Q G
≈ 30mA to 60mA (typ)
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. This architecture relies on
the output filter capacitor’s ESR to act as a current-
sense resistor, so the feedback ripple voltage provides
the PWM ramp signal. The control algorithm is simple:
the high-side switch on-time is determined solely by a
one-shot whose pulse width is inversely proportional to
input voltage and directly proportional to output volt-
age. Another one-shot sets a minimum off-time (400ns
typ). The on-time one-shot is triggered if the error com-
parator is low, the low-side switch current is below the
valley current-limit threshold, and the minimum off-time
one-shot has timed out.
On-Time One-Shot
The heart of the PWM core is the one-shot that sets the
high-side switch on-time. This fast, low-jitter, adjustable
one-shot includes circuitry that varies the on-time in
response to battery and output voltage. The high-side
switch on-time is inversely proportional to the battery
voltage as sensed by IN, and proportional to the feed-
back voltage:
where K (switching period) is set 2.5µs for side 1 and
3.3µs for side 2. For continuous conduction operation,
the actual switching frequency can be estimated by:
where V DROP1 is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PCB resistances; V
DROP2 is the
sum of the parasitic voltage drops in the charging path,
including the high-side switch, inductor, and PCB resis-
tances; and t
ON
is the on-time calculated by the
MAX17031.
f
V V
t V V V
SW
OUT DROP
ON IN DROP DROP
=
+
( )
× +
( )
−
1
1 2
t
K V
V
ON
OUT
IN
=
×
Dual Quick-PWM Step-Down Controller with Low-
Power LDO and RTC Regulator for MAIN Supplies
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