Rainbow Electronics MAX17101 - Manual (Page 18)

MAX17101
Dual Quick-PWM, Step-Down Controller
with Low-Power LDO, RTC Regulator
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Detailed Description
The MAX17101 step-down controller is ideal for high-
voltage, low-power supplies for notebook computers.
Maxim’s Quick-PWM pulse-width modulator in the
MAX17101 is specifically designed for handling fast
load steps while maintaining a relatively constant oper-
ating frequency and inductor operating point over a
wide range of input voltages. The Quick-PWM architec-
ture circumvents the poor load-transient timing prob-
lems of fixed-frequency current-mode PWMs, while also
avoiding the problems caused by widely varying
switching frequencies in conventional constant-on-time
and constant-off-time PWM schemes. Figure 2 is a
functional diagram overview. Figure 3 is the functional
diagram—QuickPWM core.
The MAX17101 includes several features for multipur-
pose notebook functionality, allowing this controller to
be used two or three times in a single notebook—main,
I/O chipset, and graphics. The MAX17101 includes a
100mA LDO that can be configured for preset 5V oper-
ation—ideal for initial power-up of the notebook and
main supply. Additionally, the MAX17101 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
MAX17101 also includes an optional secondary feed-
back input that allows an unregulated charge pump or
secondary winding to be included on a supply—ideal
for generating the low-power 12V-to-15V load switch
supply. Finally, the MAX17101 includes a reference
input on SMPS 2 that allows dynamic voltage transitions
when driven by an adjustable resistive voltage-divider or
DAC—ideal for the dynamic graphics core requirements.
3.3V RTC Power
The MAX17101 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 real-
time clock (RTC) when all other notebook regulators are
disabled. RTC also serves as the main bias supply of
the MAX17101 so it powers up before the LDC and
switching regulators. The RTC regulator sources at
least 5mA for external loads.
Adjustable 100mA Linear Regulator
The MAX17101 includes a high-current (100mA) linear
regulator that may be configured for preset 5V or 3.3V
operation. When the MAX17101 is configured as a main
supply, this LDO is required to generate the 5V bias
supply necessary to power up the switching regulators.
Once the switching regulators are enabled, the LDO
may be bypassed using the dedicated BYP input. The
adjustable linear regulator allows generation of the 3.3V
suspend supply or buffered low-power chipset and
GPU reference supplies. The MAX17101 LDO sources
at least 100mA of supply current.
Bypass Switch
The MAX17101 includes an independent LDO bypass
input that allows the LDO to be bypassed by either
switching regulator output or from a different regulator all
together. When the bypass voltage (BYP) exceeds 93.5%
of the LDO output voltage for 500μs, the MAX17101
reduces the LDO regulation threshold and turns on an
internal p-channel MOSFET to short BYP to LDO. Instead
of disabling the LDO when the MAX17101 enables the
bypass switch, the controller reduces the LDO regulation
voltage, which effectively places the linear regulator in a
standby state while switched over, yet allows a fast
recovery if the bypass supply drops.
Connect BYP to GND when not used to avoid uninten-
tional conduction through the body diode (BYP to LDO)
of the p-channel MOSFET.
5V Bias Supply (V CC /V DD )
The MAX17101 requires an external 5V bias supply
(V
DD
and V
CC
) in addition to the battery. Typically, this
5V bias supply is generated by either the internal
100mA LDO (when configured for a main supply) or
from the notebook’s 95%-efficient 5V main supply (when
configured for I/O chipset, DDR, or graphics). Keeping
these bias supply inputs independent improves the
overall efficiency and allows the internal linear regulator
to be used for other applications as well.
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 may
be estimated as follows:
I BIAS(MAX) = I CC(MAX) + f SW Q G
≈ 30mA to 60mA (typ)