Electronic Components Datasheet Search
Selected language     English  β–Ό


MAX877 Datasheet(PDF) 8 Page - Maxim Integrated Products

Part No. MAX877
Description  5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters
Download  12 Pages
Scroll/Zoom Zoom In 100% Zoom Out
Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
Logo 

 
 8 page
background image
Active Rectifier
The internal active rectifier of the MAX877/MAX878/
MAX879 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 shutdown.
The MAX877/MAX878/MAX879 can withstand a momen-
tary short at the output in normal operation.
The active rectifier also acts as a zero-dropout regulator
if the input exceeds the regulated output.The device still
switches to deliver power to the output, and the differ-
ence between the input and output voltage appears
across the rectifier. Efficiency is similar to that of a linear
regulator if the MAX877/MAX878/MAX879 are used as
step-down converters. The maximum output current
(IOUT (MAX)) with larger input/output differentials is deter-
mined by package power dissipation. IOUT (MAX) can be
approximated by:
PDISS
IOUT (MAX)
β‰ˆ
(β€”β€”β€”β€”β€”β€” )x 0.9
(VIN - VOUT)
Shutdown
Shutdown (S
β€”
H
β€”
D
β€”
N
–
) is a high-impedance, active-low
input. Connect it to IN for normal operation. Keeping
S
β€”
H
β€”
D
β€”
N
–
at ground holds the converters in shutdown
mode. Since the active rectifier is turned off in this
mode, the path from input to load is cut, and the output
effectively drops to 0V. The supply current in shutdown
mode ranges from 4Β΅A at VIN = 1V to 50Β΅A at VIN = 5V.
The shutdown-circuit threshold is set nominally to VIN/2
+ 250mV. When S
β€”
H
β€”
D
β€”
N
–
is below this threshold, the
device is shut down; it is enabled with S
β€”
H
β€”
D
β€”
N
–
above the
threshold. When driven from external logic, S
β€”
H
β€”
D
β€”
N
–
can
be driven to a higher voltage than VIN, (6.2V max).
Current Limit
Connecting ILIM to IN sets an LX current limit of 1A. For
smaller output power levels that do not require the maxi-
mum peak current, reduce the peak inductor current by
connecting a resistor between ILIM and IN. This opti-
mizes overall efficiency and allows very small, low-cost
coils with lower current ratings. See Figure 2 to select
the resistor (see also
Inductor Selection section).
Output Voltage Selection
The MAX877’s output voltage is fixed at 5V. The MAX878’s
output voltage can be set to 3V by leaving the SEL pin
open, or to 3.3V by connecting SEL to AGND.
The MAX879’s output voltage is set by two resistors, R1
and R2 (Figure 3), which form a voltage divider
between the output and the FB pin. The output voltage
can be set from 2.5V to 6.0V by the equation:
VOUT = VREF
(R1 + R2)
R2
where VREF = 0.2025V.
To simplify the resistor selection:
R1 = R2
(
VOUT
VREF
- 1
)
Since the input current at FB has a maximum of 40nA,
large values (10k
Ω to 50kΩ for R2) can be used without
significant accuracy loss. For 1% error, the current
through R2 should be at least 100 times FB’s bias current.
When large values are used for the feedback resistors
(R1 > 50k
Ω), stray output impedance at FB can add a
β€œlag” to the feedback response, destabilizing the regula-
tor and creating a larger ripple at the output. Lead
lengths and circuit board traces at the FB node should be
kept short. Reduce ripple by adding a β€œlead” compensa-
tion capacitor (C3, 100pF to 50nF) in parallel with R1.
5V/3.3V/3V/Adjustable-Output,
Step-Up/Step-Down DC-DC Converters
8
_______________________________________________________________________________________
1200
02468
10
12
14
CURRENT-LIMIT RESISTOR
vs. PEAK INDUCTOR CURRENT
RESISTOR VALUE (k
Ω)
1000
600
800
400
200
0
VIN = 2.5V
Figure 2. Current-Limit Resistor vs. Peak Inductor Current




Html Pages

1  2  3  4  5  6  7  8  9  10  11  12 


Datasheet Download



Related Electronics Part Number

Part NumberComponents DescriptionHtml ViewManufacturer
ST763A3.3V STEP DOWN CURRENT MODE PWM DC-DC CONVERTERS 1 2 3 4 5 MoreSTMicroelectronics
IMS77 Watt DC-DC Converters 1 2 3 4 Power-One
NMA-1Isolated 1W Dual Output DC-DC Converters 1 2 C&D Technologies
IRF7811WPower MOSFET for DC-DC Converters 1 2 3 4 5 MoreInternational Rectifier
MAX1674High-Efficiency Low-Supply-Current Compact Step-Up DC-DC Converters 1 2 3 4 5 MoreMaxim Integrated Products
MAX5025500kHz 36V Output SOT23 PWM Step-Up DC-DC Converters 1 2 3 4 5 MoreMaxim Integrated Products
SP6122Low Voltage Micro 8 PFET Buck Controller Ideal for 1A to 5A Small Footprint DC-DC Power Converters 1 2 3 4 5 MoreSipex Corporation
PTH05050DC-DC CONVERTERS POLA Non-isolated 1 2 3 4 Artesyn Technologies
71128Simple Solution for Dynamically Programming the Output Voltage of DC-DC Converters 1 2 3 4 5 MoreVishay Siliconix
NXI15081 A DC-DC Non-Isolated Converters 1 2 3 Artesyn Technologies

Link URL

Does ALLDATASHEET help your business so far?  [ DONATE ]  

About Alldatasheet   |   Advertisement   |   Contact us   |   Privacy Policy   |   Bookmark   |   Link Exchange   |   Manufacturer List
All Rights ReservedΒ© Alldatasheet.com 2003 - 2017    


Mirror Sites
English : Alldatasheet.com  , Alldatasheet.net  |   Chinese : Alldatasheetcn.com  |   German : Alldatasheetde.com  |   Japanese : Alldatasheet.jp  |   Russian : Alldatasheetru.com
Korean : Alldatasheet.co.kr   |   Spanish : Alldatasheet.es  |   French : Alldatasheet.fr  |   Italian : Alldatasheetit.com  |   Portuguese : Alldatasheetpt.com  |   Polish : Alldatasheet.pl