MAX1631EAI+T Maxim Integrated Products, MAX1631EAI+T Datasheet - Page 13

• Manufacturer Part Number: MAX1631EAI+T
• Description: IC PS CTRLR FOR NOTEBOOKS 28SSOP
• Manufacturer: Maxim Integrated Products
• Type: Step-Down (Buck)r

Specifications of MAX1631EAI+T

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 3.3V, 5V, Adj
• Current - Output: 4A
• Frequency - Switching: 200kHz, 300kHz
• Voltage - Input: 4.2 ~ 30 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Power - Output: 762mW
• Output Voltage: 2.5 V to 5.5 V, 3.3 V, 5 V
• Input Voltage: 4.2 V to 30 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

In PWM mode, the controller operates as a fixed-
frequency current-mode controller where the duty ratio
is set by the input/output voltage ratio. The current-
mode feedback system regulates the peak inductor
current value as a function of the output-voltage error
signal. In continuous-conduction mode, the average
inductor current is nearly the same as the peak current,
so the circuit acts as a switch-mode transconductance
amplifier. This pushes the second output LC filter pole,
normally found in a duty-factor-controlled (voltage-
mode) PWM, to a higher frequency. To preserve inner-
loop stability and eliminate regenerative inductor
current “staircasing,” a slope compensation ramp is
summed into the main PWM comparator to make the
apparent duty factor less than 50%.
The MAX1630 family uses a relatively low loop gain,
allowing the use of lower-cost output capacitors. The
relative gains of the voltage-sense and current-sense
inputs are weighted by the values of current sources
that bias three differential input stages in the main PWM
comparator (Figure 4). The relative gain of the voltage
comparator to the current comparator is internally fixed
at K = 2:1. The low loop gain results in the 2% typical
load-regulation error. The low value of loop gain helps
reduce output filter capacitor size and cost by shifting
the unity-gain crossover frequency to a lower level.
Figure 4. Main PWM Comparator Block Diagram
SLOPE COMPENSATION
CSH_
CSL_
FB_
REF
______________________________________________________________________________________
Multi-Output, Low-Noise Power-Supply
I1
Controllers for Notebook Computers
R1
I2
VL
R2
I3
The output filter capacitors (Figure 1, C1 and C2) set a
dominant pole in the feedback loop that must roll off the
loop gain to unity before encountering the zero intro-
duced by the output capacitor’s parasitic resistance
(ESR) (see Design Procedure section). A 60kHz pole-
zero cancellation filter provides additional rolloff above
the unity-gain crossover. This internal 60kHz lowpass
compensation filter cancels the zero due to filter capaci-
tor ESR. The 60kHz filter is included in the loop in both
fixed-output and adjustable-output modes.
Synchronous rectification reduces conduction losses in
the rectifier by shunting the normal Schottky catch diode
with a low-resistance MOSFET switch. Also, the synchro-
nous rectifier ensures proper start-up of the boost gate-
driver circuit. If the synchronous power MOSFETs are
omitted for cost or other reasons, replace them with a
small-signal MOSFET, such as a 2N7002.
If the circuit is operating in continuous-conduction
mode, the DL drive waveform is simply the complement
of the DH high-side drive waveform (with controlled
dead time to prevent cross-conduction or “shoot-
through”). In discontinuous (light-load) mode, the syn-
chronous switch is turned off as the inductor current
falls through zero. The synchronous rectifier works
Synchronous Rectifier Driver (DL)
UNCOMPENSATED
HIGH-SPEED
LEVEL TRANSLATOR
AND BUFFER
V
BIAS
OUTPUT DRIVER
TO PWM
LOGIC
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