MAX797CSLASHD datasheet(16/32 Pages) MAXIM

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Step-Down Controllers with

Synchronous Rectifier for CPU Power

______________________________________________________________________________________

It’s often possible to achieve a bootstrap-like effect, even

for circuits that are set to VOUT < 4.5V, by powering VL

from an external-system +5V supply. To achieve this

pseudo-bootstrap, add a Schottky diode between the

external +5V source and VL, with the cathode to the VL

side. This circuit provides a 1% to 2% efficiency boost

and also extends the minimum battery input to less than

4V. The external source must be in the range of 4.8V to

6V. Another way to achieve a pseudo-bootstrap is to add

an extra flyback winding to the main inductor to generate

the +5V bootstrap source, as shown in the +3.3V/+5V

Dual-Output Application (Figure 12).

Boost High-Side

Gate-Driver Supply (BST Pin)

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

generated by a flying-capacitor boost circuit as shown

in Figure 5. The capacitor is alternately charged from

the VL supply and placed in parallel with the high-side

MOSFET’s gate-source terminals.

On start-up, the synchronous rectifier (low-side MOS-

FET) forces LX to 0V and charges the BST capacitor to

5V. On the second half-cycle, the PWM turns on the

high-side MOSFET by closing an internal switch

between BST and DH. This provides the necessary

enhancement voltage to turn on the high-side switch,

an action that “boosts” the 5V gate-drive signal above

the battery voltage.

Ringing seen at the high-side MOSFET gate (DH) in

discontinuous-conduction mode (light loads) is a natur-

al operating condition, and is caused by the residual

energy in the tank circuit formed by the inductor and

stray capacitance at the switching node LX. The gate-

driver negative rail is referred to LX, so any ringing

there is directly coupled to the gate-drive output.

Current-Limiting and

Current-Sense Inputs (CSH and CSL)

The current-limit circuit resets the main PWM latch and

turns off the high-side MOSFET switch whenever the

voltage difference between CSH and CSL exceeds

100mV. This limiting is effective for both current flow

directions, putting the threshold limit at ±100mV. The

tolerance on the positive current limit is ±20%, so the

external low-value sense resistor must be sized for

80mV/R1 to guarantee enough load capability, while

components must be designed to withstand continuous

current stresses of 120mV/R1.

For breadboarding purposes or very high-current appli-

cations, it may be useful to wire the current-sense inputs

with a twisted pair rather than PC traces. This twisted

pair needn’t be anything special, perhaps two pieces of

wire-wrap wire twisted together.

Oscillator Frequency and

Synchronization (SYNC Pin)

The SYNC input controls the oscillator frequency.

Connecting SYNC to GND or to VL selects 150kHz

operation; connecting SYNC to REF selects 300kHz.

SYNC can also be used to synchronize with an external

5V CMOS or TTL clock generator. SYNC has a guaran-

teed 190kHz to 340kHz capture range.

300kHz operation optimizes the application circuit for

component size and cost. 150kHz operation provides

increased efficiency and improved load-transient

response at low input-output voltage differences (see

Low-Voltage Operation section).

Low-Noise Mode (SKIP Pin)

The low-noise mode (SKIP = high) is useful for minimiz-

ing RF and audio interference in noise-sensitive appli-

cations such as Soundblaster™ hi-fi audio-equipped

systems, cellular phones, RF communicating comput-

ers, and electromagnetic pen-entry systems. See the

summary of operating modes in Table 3. SKIP can be

driven from an external logic signal.

The MAX797 can reduce interference due to switching

noise by ensuring a constant switching frequency

regardless of load and line conditions, thus concentrat-

ing the emissions at a known frequency outside the

system audio or IF bands. Choose an oscillator fre-

MAX796

MAX797

MAX799

BST

+5V

VL SUPPLY

BATTERY

INPUT

PWM

LEVEL

TRANSLATOR

Figure 5. Boost Supply for Gate Drivers

Soundblaster is a trademark of Creative Labs.

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