MAX879L Maxim Integrated Products, MAX879L Datasheet - Page 7

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

The MAX777L/MAX778L/MAX779L combine a switch-
mode regulator with an NPN bipolar switch, current
limit, precision voltage reference, and active rectifier—
all in a single monolithic device. In shutdown mode,
the internal rectifier is completely turned off and dis-
connects the load from the source. Only two external
components are required in addition to the input
bypass capacitor: a 22µH inductor and a 100µF filter
capacitor.
A minimum off-time, current-limited, pulse-frequency-
modulation (PFM) control scheme combines the advan-
tages of pulse width modulation (PWM) (high output
power and efficiency) with those of a traditional PFM
pulse skipper (low quiescent currents).
External conditions (inductor value, load, and input volt-
age) determine the way the converter operates, as follows:
Figure 1. MAX778L Block Diagram
_______________Detailed Description
V
Adjustable Output, Step-Up DC-DC Converters
IN
7
SHDN
_______________________________________________________________________________________
DELAY
DELAY
TIMER
TIMER
t
t
OFF
ON
MAX778L
SHUTDOWN
CONTROL
Operating Principle
Low-Voltage Input, 3V/3.3V/5V/
VREF
0.2025V
100 F
IN
2
SWITCH
DRIVER
At light loads, the current through the inductor starts at
zero, rises to a peak value, and drops down to zero in
each cycle (discontinuous-conduction mode). In this
case, the switching frequency is governed by a pair of
one-shots that set a maximum on-time inversely propor-
tional to V IN [t
time (1.3µs for MAX777L/MAX779L and 2.3µs for
MAX778L). With a 22µH inductor, LX’s peak current is
about 400mA and is independent of input voltage.
Efficiency at light loads is improved because of lower
peak currents.
At very light loads, more energy is stored in the coil
than is required by the load in each cycle. The converter
regulates by skipping entire cycles. Efficiency is typically
65% to 75% in the pulse-skipping mode. Pulse-skipping
waveforms can be irregular, and the output waveform
contains a low-frequency component. Larger, low equiv-
alent series resistance (ESR) filter capacitors can help
reduce the ripple voltage if needed.
22 H
R
LIM
ILIM
1
1
:
LX
N
ON
ACTIVE RECTIFIER
= 8.8/(V
RECTIFIER
CONTROL
IN
AGND
- 0.25)] and a minimum off-
3
PGND
OUT
SEL 8
4
6
100 F
V
OUT
7