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PDF MAX8505 Data sheet ( Hoja de datos )
| Número de pieza | MAX8505 | |
| Descripción | Step-Down Regulator | |
| Fabricantes | Maxim Integrated | |
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19-2992; Rev 1; 9/10
EVAALVUAAILTAIOBNLEKIT
16-QSOP 3A, 1MHz, 1% Accurate, Internal Switch
Step-Down Regulator with Power-OK
m x 4.9m
General Description
The MAX8505 step-down regulator operates from a 2.6V
to 5.5V input and generates an adjustable output voltage
from 0.8V to 0.85 VIN at up to 3A. With a 2.6V to 5.5V
bias supply, the input voltage can be as low as 2.25V.
The MAX8505 integrates power MOSFETs and
operates at 1MHz/500kHz switching frequency to
provide a compact design. Current-mode pulse-width-
modulated (PWM) control simplifies compensation with
ceramic or polymer output capacitors and provides
excellent transient response.
The MAX8505 features 1% accurate output over load,
line, and temperature variations. Adjustable soft-start is
achieved with an external capacitor. During the
soft-start period, the voltage-regulation loop is active.
This limits the voltage dip when the active devices,
such as microprocessors or ASICs connected to the
MAX8505’s output, apply a sudden load current step
upon passing their undervoltage thresholds.
The MAX8505 features current-limit, short-circuit, and
thermal-overload protection and enables a rugged
design. Open-drain power-OK (POK) monitors the
output voltage.
Features
o Saves Space—4.9mm x 6mm Footprint, 1µH
Inductor, 47µF Ceramic Output Capacitor
o Input Voltage Range
2.6V to 5.5V
Down to 2.25V with Bias Supply
o 0.8V to 0.85 VIN, 3A Output
o Ceramic or Polymer Capacitors
o ±1% Output Accuracy Over Load, Line, and
Temperature
o Fast Transient Response
o Adjustable Soft-Start
o In-Regulation Soft-Start Limits Output-Voltage
Dips at Power-On
o POK Monitors Output Voltage
Ordering Information
Applications
µP/ASIC/DSP/FPGA Core and I/O Supplies
Chipset Supplies
Server, RAID, and Storage Systems
Network and Telecom Equipment
PART
TEMP RANGE PIN-PACKAGE
MAX8505EEE+
-40°C to +85°C 16 QSOP
+Denotes a lead(Pb)-free/RoHS-compliant package.
Functional Diagram appears at end of data sheet.
Pin Configuration
Typical Operating Circuit
TOP VIEW
LX 1
IN 2
LX 3
IN 4
BST 5
VCC 6
POK 7
CTL 8
MAX8505
QSOP
16 LX
15 PGND
14 LX
13 PGND
12 GND
11 REF
10 FB
9 COMP
INPUT
2.6V TO 5.5V
ENABLE
POWER-OK
BST
IN
LX
MAX8505
PGND
VCC
FB
COMP
CTL
REF
POK
GND
OUTPUT
0.8V TO
0.85 x VIN
3A
________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
1 page  
3A, 1MHz, 1% Accurate, Internal Switch
Step-Down Regulator with Power-OK
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCC = VCTL = +3.3V, VFB = 0.8V, VCOMP = 1.25V, CREF = 0.01µF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER
LX Maximum Duty Cycle
LX Minimum Duty Cycle
SLOPE COMPENSATION
Slope Compensation
BST
BST Shutdown Supply Current
CTL
CTL Input Threshold
CTL Input Current
POK (Power-OK)
POK Output Voltage, Low
POK Leakage Current
POK Fault Delay Time
SYMBOL
CONDITIONS
VIN = VCC = 2.6V, 3.3V, 500kHz
5.5V
1MHz
VIN = VCC = 2.6V, 3.3V, 500kHz
5.5V
1MHz
Extrapolated to 100% duty cycle
(VBST - VLX) = VIN =
VCC = 5.5V, VCTL = 0V
VLX = 5.5V
VLX = 0V
LX open
VIN = VCC = 2.6V,
3.3V, 5.5V
For 1MHz
For 500kHz
For shutdown
VCTL = 0V or 5.5V, VIN = VCC = 5.5V
VFB = 0.6V or 1.0V, IPOK = 2mA
VPOK = 5.5V
From FB to POK, any threshold
MIN TYP MAX UNITS
90
%
84
8
%
15
245 406 mV
10
10 µA
10
80
% of
55 70
VCC
45
-1 +1 µA
100 mV
1 µA
25 100 µs
Note 1: Under normal operating conditions, COMP moves between 1.25V and 2.15V as the duty cycle changes from 10% to 90%
and peak inductor current changes from 0 to 3A. Maximum output current is related to peak inductor current, inductor value
input voltage, and output voltage by the following equations:
IOUT _MAX
=
ILIM − (1− D) × tS × VOUT / 2L
1+ (1− D) × tS × (RNLS + RL) /2L
where VOUT = output voltage; ILIM = current limit of high-side switch; tS = switching period; RL = ESR of inductor; RNLS =
on-resistance of low-side switch; L = inductor. Equations for ILIM and D are shown as follows:
ILIM
=
ILIM _ DC100
+
VSW
1− D
RT
where ILIM_DC100 = current limit at D = 100%; RT = transresistance from LX to COMP; VSW = slope compensation (310mV
±20%); D = duty cycle:
D = VOUT + IO(RNLS + RL )
VIN + IO(RNLS − RNHS )
where VOUT = output voltage; VIN = input voltage; IO = output current; RL = ESR of inductor; RNHS = on-resistance of high-
side switch; RNLS = on-resistance of low-side switch. See the Typical Application Circuit for external components.
Note 2: Specifications to -40°C are guaranteed by design and not production tested.
Note 3: LX has internal clamp diodes to PGND and IN pins 2 and 4. Applications that forward bias these diodes should take care
not to exceed the IC’s package power dissipation limits.
Note 4: When connected together, the LX output is designed to provide 3.5ARMS current.
_______________________________________________________________________________________ 5
5 Page 
3A, 1MHz, 1% Accurate, Internal Switch
Step-Down Regulator with Power-OK
Output Voltage Selection
The output voltage of the MAX8505 can be adjusted
from 0.8V to 85% of the input voltage at 500kHz or up
to 80% of the input voltage at 1MHz. This is done by
connecting a resistive-divider (R2 and R3) between the
output and the FB pin (see the Typical Operating
Circuit). For best results, keep R3 below 50kΩ and
select R2 using the following equation:
⎛
R2 = R3 × ⎝⎜
VOUT
VREF
⎞
− 1⎠⎟
where VREF = 0.8V.
Inductor Design
When choosing the inductor, the key parameters are
inductor value (L) and peak current (IPEAK). The
following equation includes a constant, denoted as LIR,
which is the ratio of peak-to-peak inductor AC current
(ripple current) to maximum DC load current. A higher
value of LIR allows smaller inductance but results in
higher losses and ripple. A good compromise between
size and losses is found at approximately 20% to 30%
ripple-current to load-current ratio (LIR = 0.20 to 0.30):
L = VOUT × (1− D)
IOUT × LIR × fS
where fS is the switching frequency and
LIR = 2 × (IPEAK − IOUT )
IOUT
Choose an inductor with a saturation current at least as
high as the peak inductor current. Additionally, verify
the peak inductor current does not exceed the current
limit. The inductor selected should exhibit low losses at
the chosen operating frequency.
Output Capacitor Design and Output Ripple
The key selection parameters for the output capacitor
are capacitance, ESR, ESL, and the voltage rating
requirements. These affect the overall stability, output
ripple voltage, and transient response of the DC-DC
converter. The output ripple occurs due to variations in
the charge stored in the output capacitor, the voltage
drop due to the capacitor’s ESR, and the voltage drop
due to the capacitor’s ESL. Calculate the output voltage
ripple due to the output capacitance, ESR, and ESL as:
VRIPPLE = VRIPPLE(C)2 + VRIPPLE(ESR)2 + VRIPPLE(ESL)2
where the output ripples due to output capacitance,
ESR, and ESL are:
VRIPPLE(C)
=
IP − P
8 × COUT
× fS
VRIPPLE(ESR) = IP−P × ESR
VRIPPLE(ESL)
=
IP − P
tON
× ESL
or
IP−P × ESL,
tOFF
or, whichever is greater.
The ESR is the main contribution to the output voltage
ripple.
IP-P, the peak-to-peak inductor current, is:
IP − P
=
(VIN − VOUT)
fS × L
×
VOUT
VIN
Use these equations for initial capacitor selection,
but determine final values by testing a prototype or
evaluation circuit. As a rule, a smaller ripple current
results in less output voltage ripple. Since the inductor
ripple current is a factor of the inductor value, the
output voltage ripple decreases with larger inductance.
Use ceramic capacitors for their low ESR and ESL at the
switching frequency of the converter. The low ESL of
ceramic capacitors makes ripple voltages negligible.
Load-transient response depends on the selected
output capacitor. During a load transient, the output
instantly changes by ESR ILOAD. Before the controller
can respond, the output deviates further, depending on
the inductor and output capacitor values. After a short
time (see Transient Response in the Typical Operating
Characteristics), the controller responds by regulating the
output voltage back to its nominal state. The controller
response time depends on the closed-loop bandwidth,
the inductor value, and the slew rate of the transconduc-
tance amplifier. A higher bandwidth yields a faster
response time, thus preventing the output from deviating
further from its regulating value.
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet MAX8505.PDF ] | |
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