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PDF MAX1638 Data sheet ( Hoja de datos )
| Número de pieza | MAX1638 | |
| Descripción | High-Speed Step-Down Controller with Synchronous Rectification for CPU Power | |
| Fabricantes | Maxim Integrated | |
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19-1313; Rev 0; 11/97
EVAALVUAAILTAIOBNLEKIT
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
_______________General Description
The MAX1638 is an ultra-high-performance, step-down
DC-DC controller for CPU power in high-end computer
systems. Designed for demanding applications in which
output voltage precision and good transient response are
critical for proper operation, it delivers over 35A from 1.3V
to 3.5V with ±1% total accuracy from a +5V ±10% supply.
Excellent dynamic response corrects output transients
caused by the latest dynamically clocked CPUs. This
controller achieves over 90% efficiency by using synchro-
nous rectification. Flying-capacitor bootstrap circuitry
drives inexpensive, external N-channel MOSFETs.
The switching frequency is pin-selectable for 300kHz,
600kHz, or 1MHz. High switching frequencies allow the
use of a small surface-mount inductor and decrease out-
put filter capacitor requirements, reducing board area
and system cost.
The MAX1638 is available in 24-pin SSOP and QSOP
(future package) packages, and offers additional fea-
tures such as a digitally programmable output;
adjustable transient response; and selectable 0.5%, 1%,
or 2% AC load regulation. Fast recovery from load tran-
sients is ensured by a GlitchCatcher™ current-boost cir-
cuit that eliminates delays caused by the buck inductor.
Output overvoltage protection is enforced by a crowbar
circuit that turns on the low-side MOSFET with 100%
duty factor when the output is 200mV above the normal
regulation point. Other features include internal digital
soft-start, a power-good output, and a 3.5V ±1% refer-
ence output.
________________________Applications
Pentium Pro™, Pentium II™, PowerPC™, Alpha™,
and K6™ Systems
Workstations
Desktop Computers
LAN Servers
GTL Bus Termination
______________Ordering Information
PART
MAX1638EAG
MAX1638EEG*
TEMP. RANGE
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
24 SSOP
24 QSOP
Pin Configuration appears at end of data sheet.
*Future product—contact factory for package availability.
Pentium Pro and Pentium II are trademarks of Intel Corp.
PowerPC is a trademark of IBM Corp.
Alpha is a trademark of Digital Equipment Corp.
K6 is a trademark of Advanced Micro Devices.
GlitchCatcher is a trademark of Maxim Integrated Products.
____________________________Features
o Better than ±1% Output Accuracy Over
Line and Load
o Greater than 90% Efficiency Using N-Channel
MOSFETs
o Pin-Selected High Switching Frequency (300kHz,
600kHz, or 1MHz)
o Over 35A Output Current
o Digitally Programmable Output from 1.3V to 3.5V
o Current-Mode Control for Fast Transient
Response and Cycle-by-Cycle Current-Limit
Protection
o Short-Circuit Protection with Foldback Current
Limiting
o Glitch-Catcher Circuit for Fast Load-Transient
Response
o Crowbar Overvoltage Protection
o Power-Good (PWROK) Output
o Digital Soft-Start
o High-Current (2A) Drive Outputs
o Complies with Intel VRM 8.2 Specification
__________Typical Operating Circuit
VCC
AGND
VDD
BST
TO VDD MAX1638 DH
LX
PWROK
D0
D1
DL
D2
D3 PGND
D4
REF
LG
FREQ
CC1
CC2
CSH
CSL
FB
INPUT
+5V
OUTPUT
1.3V TO 4.5V
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
1 page  
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
__________________________________________Typical Operating Characteristics
(TA = +25°C, using the MAX1638 evaluation kit, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
WITHOUT GLITCHCATCHER (COUT = 880µF)
LOAD-TRANSIENT RESPONSE
WITHOUT GLITCHCATCHER (COUT = 440µF)
LOAD-TRANSIENT RESPONSE
WITH GLITCHCATCHER
A
B
10µs/div
VIN = 5V, VOUT = 2.0V, LOAD = 14A, 3A/µs
A: VOUT, 50mV/div, AC COUPLED
B: INDUCTOR CURRENT, 10A/div
FOLDBACK CURRENT LIMIT
A
A
B
10µs/div
VIN = 5V, VOUT = 2.0V, LOAD = 14A, 3A/µs
A: VOUT, 100mV/div, AC COUPLED
B: INDUCTOR CURRENT, 10A/div
START-UP WAVEFORMS
A
B
C
D
10µs/div
COUT = 440µF, VIN = 5V, VOUT = 2.0V, LOAD = 14A, 30A/µs
A: VOUT, 100mV/div,
C: NDRV, 5V/div
AC COUPLED
D: INDUCTOR CURRENT,
B: PDRV, 5V/div
10A/div
SWITCHING WAVEFORMS
A
B
A
C
BB
0
10µs/div
VO = 2.0V NOMINAL
A: VOUT = 0.5V/div
B: INDUCTOR CURRENT, 5A/div
400µs/div
A: VOUT = 0.5V/div
B: INDUCTOR CURRENT, 5A/div
1µs/div
VIN = 5V, VOUT = 2.5V, LOAD = 5A
A: LX, 5V/div
C: INDUCTOR CURRENT,
B: VOUT, 20mV/div, AC COUPLED 5A/div
EFFICIENCY vs. OUTPUT CURRENT
100
VOUT = 3.5V
90
80
70 VOUT = 1.3V
60 VOUT = 2.0V
50
40
REFERENCE VOLTAGE
vs. OUTPUT CURRENT
5.094
4.594
4.094
SINKING
CURRENT
3.594
3.094
2.594
2.094
1.594
SOURCING
CURRENT
MAXIMUM DUTY CYCLE
vs. SWITCHING FREQUENCY
100
95
90
85
80
75
70
65
60
55
30
0.1
1.094
1 10 100
0.001
0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (mA)
50
0 200 400 600 800 1000 1200
SWITCHING FREQUENCY (kHz)
_______________________________________________________________________________________ 5
5 Page 
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
100
90
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
VFB (%)
Figure 4. Foldback Current Limit
High-Side Current Sensing
The common-mode input range of the current-sense
inputs (CSH and CSL) extends to VCC, so it is possible
to configure the circuit with the current-sense resistor
on the input side rather than on the load side (Figure 5).
This configuration improves efficiency by reducing the
power dissipation in the sense resistor according to the
duty ratio.
In the high-side configuration, if the output is shorted
directly to GND through a low-resistance path, the cur-
rent-sense comparator may be unable to enforce a cur-
rent limit. Under such conditions, circuit parasitics such
as MOSFET RDS(ON) typically limit the short-circuit cur-
rent to a value around the peak-current-limit setting.
Attach a lowpass-filter network between the current-
sense pins and resistor to reduce high-frequency com-
mon-mode noise. The filter should be designed with a
time constant of around one-fifth of the on-time (130ns
at 600kHz, for example). Resistors in the 20Ω to 100Ω
range are recommended for R7 and R8. Connect the
filter capacitors C11 and C12 from VCC to CSH and
CSL, respectively.
Values of 39Ω and 3.3nF are suitable for many designs.
Place the current-sense filter network close to the IC,
within 0.1 in (2.5mm) of the CSH and CSL pins.
Overvoltage Protection
When the output exceeds the set voltage, the synchro-
nous rectifier (N2) is driven high (and N1 is driven low).
This causes the inductor to quickly dissipate any stored
energy and force the fault current to flow to ground.
Current is limited by the source impedance and para-
sitic resistance of the current path, so a fuse is required
in series with the +5V input to protect against low-
impedance faults, such as a shorted high-side MOS-
FET. Otherwise, the low-side MOSFET will eventually
fail. DL will go low if the input voltage drops below the
undervoltage lockout point.
Internal Soft-Start
Soft-start allows a gradual increase of the internal cur-
rent limit at start-up to reduce input surge currents. An
internal DAC raises the current-limit threshold from 0V
to 100mV in four steps (25mV, 50mV, 75mV, and
100mV) over the span of 1536 oscillator cycles.
__________________Design Procedure
Setting the Output Voltage
Select the output voltage using the D0–D4 pins. The
MAX1638 uses an internal 5-bit DAC as a feedback-
resistor voltage divider. The output voltage can be digi-
tally set from 1.3V to 3.5V using the D0–D4 inputs
(Table 2).
D0–D4 are logic inputs and accept both TTL and
CMOS voltage levels. The MAX1638 has both FB and
AGND inputs, allowing a Kelvin connection for remote
voltage and ground sensing to eliminate the effects of
trace resistance on the feedback voltage. (See PC
Board Layout Considerations for further details.) FB
input current is 0.1µA (max).
The MAX1638 DAC codes (D0–D4) were designed for
compatibility with the Intel VRM 8.2 specification for
output voltages between 1.8V (code 00101) and 3.5V
(code 10000). Codes 00110 to 01111 have also been
designed for 50mV increments, allowing set voltages
down to 1.300V. Code 11111 turns off the buck controller,
placing the IC in a shutdown mode (0.2mA typical).
Choosing the
Error-Amplifier Gain
Set the error-amplifier gain to match the voltage-preci-
sion requirements of the CPU used. The MAX1638’s
loop-gain control input (LG) allows trade-offs in DC/AC
voltage accuracy versus output filter capacitor require-
ments. AC load regulation can be set to 0.5%, 1%,
or 2% by connecting LG as shown in Table 3.
DC load regulation is typically 10 times better than AC
load regulation, and is determined by the gain set by
the LG pin.
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet MAX1638.PDF ] | |
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