MAX1981A Maxim Integrated Products, MAX1981A Datasheet - Page 19

• Manufacturer Part Number: MAX1981A
• Description: (MAX1907A / MAX1981A) Quick-PWM Master Controllers
• Manufacturer: Maxim Integrated Products

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The MAX1907A/MAX1981A require 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 associated
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 supply can be
generated with an external linear regulator.
The 5V bias supply must provide V
and V
drawn is:
where I
cy, and Q
sheet’s total, gate-charge specification limits at V
5V.
V+ and V
source is a fixed 4.5V to 5.5V supply. If the 5V bias
supply is powered up prior to the battery supply, the
enable signal (SHDN going from low to high) must be
delayed until the battery voltage is present to ensure
startup.
Table 2. Component Suppliers
BI Technologies
Central Semiconductor
Coilcraft
Coiltronics
Fairchild Semiconductor
International Rectifier
Kemet
Panasonic
Sanyo
Siliconix (Vishay)
Sumida
Taiyo Yuden
TDK
Toko
Positioned CPU Core Power Supplies (IMVP-IV)
DD
I
BIAS
CC
(gate-drive power), so the maximum current
DD
G(LOW)
is 1.3mA (typ), f
= I
= 10mA to 60mA (typ)
SUPPLIER
Quick-PWM Master Controllers for Voltage-
can be tied together if the input power
CC
5V Bias Supply (V
+ f
______________________________________________________________________________________
and Q
SW
Detailed Description
(Q
G(HIGH)
G(LOW)
SW
is the switching frequen-
are the MOSFET data
+ Q
CC
408-749-9714 (USA) 65-281-3226 (Singapore)
G(HIGH)
847-803-6100 (USA) 81-3-5201-7241 (Japan)
(PWM controller)
408-573-4150 (USA) 03-3667-3408 (Japan)
CC
and V
)
714-447-2345 (USA)
631-435-1110 (USA)
800-322-2645 (USA)
561-752-5000 (USA)
888-522-5372 (USA)
310-322-3331 (USA)
408-986-0424 (USA)
847-468-5624 (USA)
203-268-6261 (USA)
408-982-9660 (USA)
858-675-8013 (USA)
GS
DD
=
)
PHONE
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 propor-
tional to input voltage and directly proportional to out-
put voltage. Another one-shot sets a minimum off-time
(400ns, typ). The on-time one-shot is triggered if the
error comparator is low, the low-side switch current is
below the current-limit threshold, and the minimum off-
time one-shot has timed out.
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 measured by the V+ input, and proportional
to the output voltage:
where K is set by the TON pin-strap connection (Table
3) and 0.075V is an approximation to accommodate the
Free-Running, Constant On-Time PWM
Controller with Input Feed-Forward
t
ON
= K(V
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www.centralsemi.com
www.coilcraft.com
www.coiltronics.com
www.fairchildsemi.com
www.irf.com
www.kemet.com
www.panasonic.com
www.secc.co.jp
www.vishay.com
www.sumida.com
www.t-yuden.com
www.component.tdk.com
www.tokoam.com
FB
On-Time One-Shot (TON)
+ 0.075V) / V
WEBSITE
IN
19