MAX1534 Datasheet, PDF (9/16 Pages) Maxim Integrated Products – High-Efficiency, Triple-Output, Keep-Alive Power Supply for Notebook Computers

High-Efficiency, Triple-Output, Keep-Alive
Power Supply for Notebook Computers
0.01µF
100kΩ
V_OUT1 = +3.3V ALWAYS
V_OUT2 = +1.8V ALWAYS
2.2µF 2.2µF
SHDN
POK
BP
IN
LX
PRESET _MAX1534
ILIM
FB3
FB1
FB2
OUT1
OUT2
LDOIN
GND
V_IN = +7V TO +24V
10µF
L1
D1
V_OUT3 = +5V ALWAYS
C_OUT3
V_OUT3 = +5V ALWAYS
2.2µF
NOTE: SEE TABLE 1 FOR RECOMMENDED COMPONENT VALUES. SEE TABLE 2 FOR COMPONENT SUPPLIERS.
Figure 1. MAX1534 Typical Application Circuit
The MAX1534 PFM step-down topology consumes less
power than the traditional linear regulator solution when
converting from a high-input voltage source.
Buck Converter
Current-Limited Control Architecture
The MAX1534’s buck converter uses a proprietary cur-
rent-limited control scheme with operation to 100% duty
cycle. This DC-to-DC converter pulses as needed to
maintain regulation, resulting in a variable switching fre-
quency that increases with the load. This eliminates the
high supply currents associated with conventional con-
stant-frequency pulse-width-modulation (PWM) con-
trollers that switch the MOSFET unnecessarily.
When the output voltage is too low, the error comparator
sets a flip-flop, which turns on the internal P-channel
MOSFET and begins a switching cycle (Figure 2). As
shown in Figure 3, the inductor current ramps up linearly,
storing energy in a magnetic field while charging the out-
put capacitor and servicing the load. The MOSFET turns
off when the peak current limit is reached, or when the
maximum on-time of 10µs is exceeded and the output
voltage is in regulation. If the output is out of regulation
and the peak current is never reached, the MOSFET
remains on, allowing a duty cycle up to 100%. This fea-
ture ensures the lowest possible dropout voltage. Once
the MOSFET turns off, the flip-flop resets, the inductor
current is pulled through D1, and the current through the
inductor ramps back down, transferring the stored ener-
gy to the output capacitor and load. The MOSFET
remains off until the 0.42µs minimum off-time expires,
and the output voltage drops out of regulation.
Current Limit (ILIM)
The MAX1534’s buck converter has an adjustable peak
current limit. Configure this peak current limit by con-
necting ILIM as shown in Table 3. Choose a current
limit that realistically reflects the maximum load current.
The maximum output current is half the peak current
limit. Although choosing a lower current limit allows
using an inductor with a lower current rating, it requires
a higher inductance (see Inductor Selection) and does
little to reduce inductor package size.
ILIM can be dynamically switched to achieve the high-
est efficiency over the load range. (See Buck Efficiency
vs. Load Current (Circuit 1) in the Typical Operating
Characteristics.
Linear Regulators
Internal P-Channel Pass Transistor
The MAX1534 features two 1.5Ω P-channel MOSFET
pass transistors. A P-channel MOSFET provides sever-
al advantages over similar designs using PNP pass
transistors, including longer battery life. It requires no
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