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PDF MAX1845 Data sheet ( Hoja de datos )
| Número de pieza | MAX1845 | |
| Descripción | Dual / High-Efficiency / Step-Down Controller with Accurate Current Limit | |
| Fabricantes | Maxim Integrated | |
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19-1955; Rev 2; 1/03
EVAALVUAAILTAIOBNLEKIT
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
General Description
The MAX1845 is a dual PWM controller configured for
step-down (buck) topologies that provides high efficien-
cy, excellent transient response, and high DC output
accuracy necessary for stepping down high-voltage bat-
teries to generate low-voltage chipset and RAM power
supplies in notebook computers. The CS_ inputs can be
used with low-side sense resistors to provide accurate
current limits or can be connected to LX_, using low-side
MOSFETs as current-sense elements.
The on-demand PWM controllers are free running, con-
stant on-time with input feed-forward. This configuration
provides ultra-fast transient response, wide input-output
differential range, low supply current, and tight load-reg-
ulation characteristics. The MAX1845 is simple and easy
to compensate.
Single-stage buck conversion allows the MAX1845 to
directly step down high-voltage batteries for the highest
possible efficiency. Alternatively, two-stage conversion
(stepping down the 5V system supply instead of the bat-
tery at a higher switching frequency) allows the minimum
possible physical size.
The MAX1845 is intended for generating chipset, DRAM,
CPU I/O, or other low-voltage supplies down to 1V. For a
single-output version, refer to the MAX1844 data sheet.
The MAX1845 is available in 28-pin QSOP and 36-pin
thin QFN packages.
Features
o Ultra-High Efficiency
o Accurate Current-Limit Option
o Quick-PWM™ with 100ns Load-Step Response
o 1% VOUT Accuracy over Line and Load
o Dual Mode™ Fixed 1.8V/1.5V/Adj or 2.5V/Adj Outputs
o Adjustable 1V to 5.5V Output Range
o 2V to 28V Battery Input Range
o 200/300/420/540kHz Nominal Switching Frequency
o Adjustable Overvoltage Protection
o 1.7ms Digital Soft-Start
o Drives Large Synchronous-Rectifier FETs
o Power-Good Window Comparator
o 2V ±1% Reference Output
PART
MAX1845EEI
MAX1845ETX
Ordering Information
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
28 QSOP
36 Thin QFN
6mm ✕ 6mm
Applications
Notebook Computers
CPU Core Supplies
Chipset/RAM Supply as Low as 1V
1.8V and 2.5V I/O Supplies
5V INPUT
OUTPUT1
1.8V
Pin Configurations appear at end of data sheet.
Quick-PWM and Dual Mode are trademarks of Maxim Integrated
Products.
Minimal Operating Circuit
VDD
VCC
ILIM1
ILIM2
ON1
ON2
BST1
V+
UVP
OVP
BST2
DH1 DH2
LX1 LX2
DL1 DL2
TON
CS1
OUT1
PGOOD
REF
CS2
OUT2
SKIP
FB1 FB2
GND
BATTERY
4.5V TO 28V
MAX1845EEI
OUTPUT2
2.5V
________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1 page  
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VDD = VCC = 5V, SKIP = AGND, V+ = 15V, TA = -40°C to +85°C, unless otherwise noted.) (Note 5)
PARAMETER
PWM CONTROLLERS
Input Voltage Range
DC Output Voltage, OUT1 (Note 2)
DC Output Voltage, OUT2 (Note 2)
Output Voltage Adjust Range
Dual-Mode Threshold (Low)
SYMBOL
CONDITIONS
V+ Battery voltage, V+
VCC/VDD VCC, VDD
VOUT1
V+ = 2V to 28V, SKIP = VCC,
ILOAD = 0 to 10A
VOUT2
V+ = 2V to 28V, SKIP = VCC,
ILOAD = 0 to 10A
OUT1, OUT2
OVP, FB_
FB1 to AGND
FB1 to VCC
FB1 to OUT1
FB2 to AGND
FB2 to OUT2
Dual-Mode Threshold (High)
OUT_ Input Resistance
FB_ Input Bias Current
On-Time, Side 1 (Note 3)
On-Time, Side 2 (Note 3)
On-Time Tracking (Note 3)
Minimum Off-Time (Note 3)
Quiescent Supply Current (VCC)
Quiescent Supply Current (VDD)
Quiescent Supply Current (V+)
Reference Voltage
Reference Load Regulation
Overvoltage Trip Threshold
(Fixed-Threshold Mode)
OVP, ILIM_
ROUT1
ROUT2
IFB
tON1
tON2
tOFF
ICC
IDD
I+
VREF
FB_
VOUT1 = 1.5V
VOUT2 = 2.5V
V+ = 24V, VOUT1 = 2V
V+ = 24V, VOUT2 = 2V
On-time 2, with
respect to on-time 1
TON = AGND
TON = REF
TON = float
TON = VCC
TON = AGND
TON = REF
TON = float
TON = VCC
TON = AGND
TON = REF
TON = float
TON = VCC
FB forced above the regulation point
FB forced above the regulation point
Measured at V+
VCC = 4.5V to 5.5V, no external REF load
IREF = 0 to 50uA
OVP = GND, with respect to FB_ regulation
point, no load
MIN
2
4.5
1.773
1.477
0.985
2.463
0.985
1
0.05
VCC -
1.5
1.9
75
100
-0.1
120
153
217
308
160
205
295
422
125
125
125
125
1.98
112
TYP
MAX
28
5.5
1.827
1.523
1.015
2.537
1.015
5.5
0.15
VCC -
0.4
2.1
0.1
153
195
272
390
204
263
371
534
145
145
145
145
500
1500
5
70
2.02
0.01
117
Output Undervoltage Threshold
Current-Limit Threshold (Fixed)
UVP = VCC, with respect to FB_ regulation
point, no load
AGND - VCS_, ILIM_ = VCC
65
35
75
65
UNITS
V
V
V
V
V
V
kΩ
µA
ns
ns
%
ns
µA
µA
µA
V
V
%
%
mV
_______________________________________________________________________________________ 5
5 Page 
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
time is determined solely by a one-shot whose pulse
width is inversely proportional to input voltage and
directly proportional to output 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
(Table 3).
On-Time One-Shot (TON)
The heart of the PWM core is the one-shot that sets the
high-side switch on-time for both controllers. 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 pro-
portional to the battery voltage as measured by the V+
input, and proportional to the output voltage. This algo-
rithm results in a nearly constant switching frequency
despite the lack of a fixed-frequency clock generator.
The benefits of a constant switching frequency are
twofold: First, the frequency can be selected to avoid
noise-sensitive regions such as the 455kHz IF band;
second, the inductor ripple-current operating point
remains relatively constant, resulting in easy design
methodology and predictable output voltage ripple.
The on-times for side 1 are set 35% higher than the on-
times for side 2. This is done to prevent audio-frequen-
cy “beating” between the two sides, which switch
asynchronously for each side. The on-time is given by:
On-Time = K (VOUT + 0.075V) / VIN
where K is set by the TON pin-strap connection (Table
4), and 0.075V is an approximation to accommodate
for the expected drop across the low-side MOSFET
switch. One-shot timing error increases for the shorter
on-time settings due to fixed propagation delays; it is
approximately ±12.5% at higher frequencies and ±10%
at lower frequencies. This translates to reduced switch-
ing-frequency accuracy at higher frequencies (Table
4). Switching frequency increases as a function of load
current due to the increasing drop across the low-side
MOSFET, which causes a faster inductor-current dis-
charge ramp. The on-times guaranteed in the Electrical
Characteristics tables are influenced by switching
delays in the external high-side power MOSFET.
VDD = 5V
BIAS SUPPLY
C9
4.7µF
C8
1µF
R1
20Ω
C11
1µF
C1
3 ✕ 10µF
OUTPUT1
1.8V, 8A
L1
2.2µH
C3
3 ✕ 470µF
D1
Q1
Q2
C5
0.1µF
R1 C7
5mΩ 0.22µF
D3
CMPSH-3A
4
21
9
22
VDD
UVP
VCC
V+
ON1 11
3 ILIM1
13
ILIM2
12
ON2
8
OVP
MAX1845EEI
25
BST1
BST2 19
26
DH1
18
DH2
27
LX1
24
DL1
5 TON
28 CS1
1 OUT1
10
REF
2 FB1
23 GND
LX2 17
DL2 20
CS2 16
OUT2 15
SKIP 6
FB2 14
PGOOD 7
C6
0.1µF
Q3
Q4
5V
100kΩ
VIN
7V TO 24V
ON/OFF
CONTROLS
C2
2 ✕ 10µF
L2
4.7µH
D2
OUTPUT2
2.5V, 4A
C4
470µF
R2
10mΩ
POWER-GOOD
INDICATOR
Figure 1. Standard Application Circuit
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 27 Páginas | |
| PDF Descargar | [ Datasheet MAX1845.PDF ] | |
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