Gate-Driver Boost Supply

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

generated with the flying-capacitor boost circuit shown

in Figure 4. The capacitor (C3) is alternately charged

from the 5V input via the diode (D1) and placed in par-

allel with the high-side MOSFET’s gate-source termi-

nals. On start-up, the synchronous rectifier (low-side)

MOSFET (N2) forces LX to 0V and charges the BST

capacitor to 5V. On the second half-cycle, the PWM

turns on the high-side MOSFET (N1); it does this by

closing an internal switch between BST and DH, which

connects the capacitor to the MOSFET gate. This pro-

vides the necessary enhancement voltage to turn on

the high-side switch, an action that “boosts” the 5V

gate-drive signal above the input voltage.

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

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

al operating condition. It is caused by the residual

energy in the tank circuit, formed by the inductor and

stray capacitance at the LX node. The gate-driver neg-

ative rail is referred to LX, so any ringing there is direct-

ly coupled to the gate-drive supply.

Modes of Operation

PWM Mode

Under heavy loads—over approximately 25% of full

load—the supply operates as a continuous-current

PWM supply (see

Typical Operating Characteristics).

The duty cycle, %ON, is approximately:

%ON = ________

Current flows continuously in the inductor: first, it ramps

up when the power MOSFET conducts; second, it

ramps down during the flyback portion of each cycle as

energy is put into the inductor and then discharged into

the load. Note that the current flowing into the inductor

when it is being charged is also flowing into the load,

so the load is continuously receiving current from the

inductor. This minimizes output ripple and maximizes

inductor use, allowing very small physical and electrical

sizes. Output ripple is primarily a function of the filter

capacitor’s effective series resistance (ESR), and is

typically under 50mV (see

Design Procedure section).

Idle-Mode

Under light loads (<25% of full load), the MAX767

enhances efficiency by turning the drive voltage on and

off for only a single clock period, skipping most of the

clock pulses entirely. Asynchronous switching, seen as

“ghosting” on an oscilloscope, is thus a normal operat-

ing condition whenever the load current is less than

approximately 25% of full load.

At certain input voltage and load conditions, a transition

region exists where the controller can pass back and

forth from idle-mode to PWM mode. In this situation,

short pulse bursts occur, which make the current wave-

form look erratic but do not materially affect the output

ripple. Efficiency remains high.

Current Limiting

The voltage between CS and FB is continuously moni-

tored. An external, low-value shunt resistor is connect-

ed between these pins, in series with the inductor,

allowing the inductor current to be continuously mea-

sured throughout the switching cycle. Whenever this

voltage exceeds 100mV, the drive voltage to the exter-

nal high-side MOSFET is cut off. This protects the MOS-

FET, the load, and the input supply in case of short cir-

cuits or temporary load surges. The current-limiting

resistance is typically 20m

Ω for 3A.

5V-to-3.3V, Synchronous, Step-Down

Power-Supply Controller

10

______________________________________________________________________________________

LEVEL

TRANSLATOR

PWM

C1

BST

DH

LX

DL

N1

D1

C3

N2

L1

MAX767

Figure 4. Boost Supply for High-Side Gate Driver