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MAX1638
(Rev.:2005)

High-Speed Step-Down Controller with Synchronous Rectification for CPU Power

Maxim Integrated 

High-Speed Step-Down Controller with

Synchronous Rectification for CPU Power

Synchronous-Rectifier Driver

Synchronous rectification reduces conduction losses in

the rectifier by shunting the normal Schottky diode or

MOSFET body diode with a low-on-resistance MOSFET

switch. The synchronous rectifier also ensures proper

start-up by precharging the boost-charge pump used

for the high-side switch gate-drive circuit. Thus, if you

must omit the synchronous power MOSFET for cost or

other reasons, replace it with a small-signal MOSFET,

such as a 2N7002.

The DL drive waveform is simply the complement of

the DH high-side drive waveform (with typical con-

trolled dead time of 30ns to prevent cross-conduction

or shoot-through). The DL output’s on-resistance is

0.7Ω (typ) and 2Ω (max).

BST High-Side Gate-Driver Supply

and MOSFET Drivers

Gate-drive voltage for the high-side N-channel switch

is generated using a flying-capacitor boost circuit

(Figure 3). The capacitor is alternately charged from

the +5V supply and placed in parallel with the high-

side MOSFET’s gate and source terminals.

Gate-drive resistors (R3 and R4) can often be useful to

reduce jitter in the switching waveforms by slowing

down the fast-slewing LX node and reducing ground

bounce at the controller IC. However, switching loss

may increase. Low-value resistors from around 1Ω to

5Ω are sufficient for many applications.

GlitchCatcher

Current-Boost Driver

Drivers for an optional GlitchCatcher current-boost cir-

cuit are included in the MAX1638 to improve transient

response in applications where several amperes of

load current are required in a matter of a few tens of

nanoseconds. The GlitchCatcher can be used to offset

the fast drop in output voltage due to the ESR of the

output capacitance. The current-boost circuit improves

transient response by providing a direct path from the

input to the output that circumvents the buck inductor’s

filtering action. When the output drops out of regulation

by more than 2%, the P-channel or N-channel switch

turns on and injects current directly into the output from

VIN or ground, forcing the output back into regulation.

The driver’s response time is typically 75ns, and mini-

mum on-time is typically 100ns. GlitchCatcher provides

the greatest benefit when the output voltage is less

than 2V, and in applications using minimum output

capacitance.

LEVEL

TRANSLATOR

CONTROL AND

DRIVE LOGIC

MAX1638

BST

DH

R4

LX

VDD

DL

R3

PGND

VIN = 5V

D2

N1 C3

N2

C1

L1

R3 AND R4

ARE OPTIONAL

Figure 3. Boost Supply for Gate Drivers

Current Sense and Overload

Current Limiting

The current-sense circuit resets the main PWM latch

and turns off the high-side MOSFET switch whenever

the voltage difference between CSH and CSL from cur-

rent through the sense resistor (R1) exceeds the peak

current limit (100mV typical).

Current-mode control provides cycle-by-cycle current-

limit capability for maximum overload protection.

During normal operation, the peak current limit set by

the current-sense resistor determines the maximum

output current. When the output is shorted, the peak

current may be higher than the set current limit due to

delays in the current-sense comparator. Thus, foldback

current limiting is employed where the set current-limit

point is reduced from 100mV to 38mV as the output

(feedback) voltage falls (Figure 4). When the short-cir-

cuit condition is removed, the feedback voltage will

rise and the current-limit voltage will revert to 100mV.

The foldback current-limit circuit is designed to ensure

startup into a resistive load.

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