MAX879L Maxim Integrated Products, MAX879L Datasheet - Page 8

• Manufacturer Part Number: MAX879L
• Description: (MAX877L - MAX879L) Step-Up & Down DC-DC Converters
• Manufacturer: Maxim Integrated Products

At heavy loads above approximately 100mA, the con-
verter enters continuous-conduction mode, where cur-
rent always flows in the inductor. The switch-on state is
controlled cycle-by-cycle by either the maximum t
time or the switch’s preset current limit. As a result, the
switch's current rating is not exceeded and the inductor
is not saturated. At very heavy loads, the inductor cur-
rent self-oscillates between this peak current limit and
some lower value governed by the minimum off-time,
the inductance value, and the input/output differential.
With ILIM shorted to IN, the peak switch current of the
internal NPN power switch is set to 1A. The peak switch
current can be set to a lower value by connecting a
resistor between ILIM and IN (see Current Limit sec-
tion). This enables the use of physically smaller induc-
tors with lower saturation-current ratings. At 1A, the
switch voltage drop (V
decreases to about 250mV at 0.1A.
Conventional PWM converters generate constant-fre-
quency switching noise, while this architecture pro-
duces variable-frequency switching noise. However,
the noise does not exceed the current limit times the fil-
ter-capacitor ESR, unlike conventional pulse-skippers.
If the input voltage exceeds the output voltage, the
MAX777L/MAX778L/MAX779L behave as “switched”
linear regulators. If the output voltage starts to drop, the
switch turns on and energy is stored in the coil, as in
normal step-up mode. After the switch turns off, the
voltage at LX flies high. The active rectifier turns on
when LX rises above V
voltage difference between V
across the rectifier (actually a PNP transistor) until the
current goes to zero and the rectifier turns off. At high
V
limited by the maximum allowable power dissipation in
the package. For fully specified buck/boost converters,
refer to the data sheet for the pin-compatible
MAX877L/MAX878L/MAX879L.
The internal active rectifier of the MAX777L/MAX778L/
MAX779L replaces the external Schottky catch diode in
normal boost operation. The rectifier consists of a PNP
pass transistor and a unique control circuit which, in
shutdown mode, entirely disconnects the load from the
source. This is a distinct advantage over standard
boost topologies, since it prevents battery drain in shut-
down.
The active rectifier also acts as a zero-dropout regulator
if the input exceeds the regulated output. This allows the
MAX777L/MAX778L/MAX779L to act as buck/boost
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters
8
IN
_______________________________________________________________________________________
to V
OUT
differentials, the maximum load current is
IN
. As in a linear regulator, the
SW
) is about 500mV. V
IN
Step-Down Mode
and V
Active Rectifier
OUT
appears
ON
SW
converters. Useful in battery-powered applications,
where the battery voltage may initially exceed the output
voltage, the converters will regulate down to the output
voltage and seamlessly switch into boost mode as the
input drops below the output voltage. The pin-compati-
ble MAX877L/MAX878L/MAX879L are fully specified
buck/boost converters with higher specified output cur-
rents than the MAX777L/MAX778L/MAX779L.
Shutdown (SHDN) is a high-impedance, active-low
input. Connect SHDN to V
Keeping SHDN at ground holds the converters in shut-
down mode. Since the active rectifier is turned off in
shutdown mode, the path from input to load is cut, and
the output effectively drops to 0V. The supply current in
the shutdown state ranges from 4µA at V
50µA at V
set nominally to V
this threshold, the device is shut down and is enabled
with SHDN above the threshold. When driven from
external logic, SHDN can be driven to a higher voltage
than V
Connecting ILIM to IN sets an LX current limit of 1A. For
smaller output power levels that do not require the max-
imum peak current, the peak inductor current can be
reduced to optimize overall efficiency and to allow very
small, low-cost coils with lower current ratings. See also
the Inductor Selection section.
Reduce the MAX777L/MAX778L/MAX779L peak induc-
tor current by connecting a resistor between ILIM and
IN. See Figure 2 to select the resistor.
Figure 2. Current-Limit Resistor vs. Current Limit
IN
.
IN
1200
1000
800
600
400
200
= 4.5V. The shutdown circuit threshold is
0
0
IN
vs. PEAK INDUCTOR CURRENT
2
CURRENT-LIMIT RESISTOR
/2 + 250mV. When SHDN is below
RESISTOR VALUE (k )
4
6
IN
8
for normal operation.
V
10
IN
= 2.5V
12
Current Limit
14
IN
Shutdown
= 1V to