MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
_______________________________________________________________________________________ 9
PWM Controller Block and Integrator
The heart of the current-mode PWM controller is a
multi-input open-loop comparator that sums three sig-
nals (Figure 2): the buffered feedback signal, the cur-
rent-sense signal, and the slope-compensation ramp.
This direct-summing configuration approaches ideal
cycle-by-cycle control over the output voltage. The out-
put voltage error signal is generated by an error ampli-
fier that compares the amplified feedback voltage to an
internal reference.
Each pulse from the oscillator sets the main PWM latch
that turns on the high-side switch for a period deter-
mined by the duty factor (approximately V
OUT
/ V
IN
).
The current-mode feedback system regulates the peak
inductor current as a function of the output voltage
error signal. Since average inductor current is nearly
the same as peak current (assuming the inductor value
is set relatively high to minimize ripple current), the cir-
cuit acts as a switch-mode transconductance amplifier.
It 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. Under fault conditions where the
inductor current exceeds the maximum current-limit
threshold, the high-side latch resets, and the high-side
switch turns off.
Internal Reference
The internal 3.5V reference (REF) is accurate to ±1%
from 0°C to +85°C, making REF useful as a system ref-
erence. Bypass REF to AGND with a 0.1µF (min)
ceramic capacitor. A larger value (such as 2.2µF) is
recommended for high-current applications. Load reg-
ulation is 10mV for loads up to 100µA. Reference
undervoltage lockout is between 2.7V and 3V. Short-
circuit current is less than 4mA.
Table 1. Component List for Standard Applications
C2
(x4) Sanyo OS-CON 4SP220M
(220μF)
Central Semiconductor CMPSH-3
Coiltronics UP4-R47
(0.47μH, 19A, SMD) or
Panasonic ETQP1F0R7H
(0.70µH, 19A, 1.6mΩ, SMD)
Int’l Rectifier IRF7307 (0.09Ω/0.05Ω)
(x2) Dale WSL-2512-R009-F (10mΩ)
(x3) Sanyo OS-CON 10SA220M
(220μF)
Int’l Rectifier IRF7307 (0.09Ω/0.05Ω)
(x6) Sanyo OS-CON 4SP220M
(220µF)
Central Semiconductor CMPSH-3
Panasonic ETQP2F1R0S
(0.70µH, 23A, 0.94mΩ, SMD)
LOAD REQUIREMENT
(x2) Dale WSR-20.007 ±1% (7mΩ)
(x4) Sanyo/OS-CON 10SA220M
(220µF)
D1
(optional)
Nihon NSQ03A02
Schottky
diode or Motorola MBRS340
Nihon NSQ03A02
Schottky
diode or Motorola MBRS340
Fairchild FDB7030L (10mΩ) or
Int’l Rectifier IRL3803S (9mΩ)
(x2) Fairchild FDB7030L (10mΩ) or
(x2) Int’l Rectifier IRL3803S (9mΩ)
N1
Fairchild FDB7030L (10mΩ) or
Int’l Rectifier IRL3803S (9mΩ)
D2
(x2) Fairchild FDB7030L (10mΩ) or
(x2) Int’l Rectifier IRL3803S (9mΩ)
N2
2.0V, 14A 2.0V, 19A
(x7) Sanyo OS-CON 4SP220M
(220µF)
Central Semiconductor CMPSH-3
Panasonic ETQP2F1R0S
(0.70µH, 23A, 0.94mΩ, SMD)
Int’l Rectifier IRF7105 (0.4Ω/0.16Ω)
(x2) Dale WSR-20.007 ±1% (7mΩ)
Dale WSL-2512-R120-J (120mΩ) —R2 (optional)
L1
(x4) Sanyo/OS-CON 10SA220M
(220µF)
P1/N3
(optional)
Nihon NSQ03A02
Schottky
diode or Motorola MBRS340
(x2) Fairchild FDB7030L (10mΩ) or
(x2) Int’l Rectifier IRL3803S (9mΩ)
(x2) Fairchild FDB7030L (10mΩ) or
(x2) Int’l Rectifier IRL3803S (9mΩ)
1.3V, 19A
—
R1
COMPONENT
C1
Note: Parts used in evaluation board are shown in bold.
