Dual-Output Power-Supply

Controller for Notebook Computers

12

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Modes of Operation

PWM Mode

Under heavy loads — over approximately 25% of full load

— the +3.3V and +5V supplies operate as continuous-

current PWM supplies (see

Typical Operating Char-

acteristics). The duty cycle (%ON) is approximately:

Current flows continuously in the inductor: First, it

ramps up when the power MOSFET conducts; then, it

ramps down during the flyback portion of each cycle

as energy is put into the inductor and then dis-

charged into the load. Note that the current flowing

into the inductor when it is being charged is also flow-

ing into the load, so the load is continuously receiving

current from the inductor. This minimizes output rip-

ple and maximizes inductor use, allowing very small

physical and electrical sizes. Output ripple is primarily

a function of the filter capacitor (C7 or C6) effective

series resistance (ESR) and is typically under 50mV

(see the

Design Procedure section). Output ripple is

worst at light load and maximum input voltage.

Idle Mode

Under light loads (<25% of full load), efficiency is fur-

ther enhanced 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 operating

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 bursts of pulses occur that make the current

waveform look erratic, but do not materially affect the

output ripple. Efficiency remains high.

Current Limiting

The voltage between CS3 (CS5) and FB3 (FB5) is contin-

uously monitored. An external, low-value shunt resistor

is connected between these pins, in series with the

inductor, allowing the inductor current to be continuously

measured throughout the switching cycle. Whenever this

voltage exceeds 100mV, the drive voltage to the external

high-side MOSFET is cut off. This protects the MOSFET,

the load, and the battery in case of short circuits or tem-

porary load surges. The current-limiting resistors R1 and

R2 are typically 25m

Ω for 3A load current.

Oscillator Frequency; SYNC Input

The SYNC input controls the oscillator frequency.

Connecting SYNC to GND or to VL selects 200kHz opera-

tion; connecting to REF selects 300kHz operation. SYNC

can also be driven with an external 240kHz to 350kHz

CMOS/TTL source to synchronize the internal oscillator.

Normally, 300kHz is used to minimize the inductor and

filter capacitor sizes, but 200kHz may be necessary for

low input voltages (see

Low-Voltage (6-Cell) Operation).

Comparators

Two noninverting comparators can be used as

precision voltage comparators or high-side drivers. The

supply for these comparators (VH) is brought out and may

be connected to any voltage between +3V and +19V

irrespective of V+. The noninverting inputs (D1-D2) are

high impedance, and the inverting input is internally con-

nected to a 1.650V reference. Each output (Q1-Q2)

sources 20µA from VH when its input is above 1.650V, and

sinks 500µA to GND when its input is below 1.650V. The

Q1-Q2 outputs can be fixed together in wired-OR

configuration since the pull-up current is only 20µA.

Connecting VH to a logic supply (5V or 3V) allows the

comparators to be used as low-battery detectors. For

driving N-channel power MOSFETs to turn external

loads on and off, VH should be 6V to 12V higher than

the load voltage. This enables the MOSFETs to be fully

The comparators are always active when V+ is above

+4V, even when VH is 0V. Thus, Q1-Q2 will sink current

to GND even when VH is 0V, but they will only source

current from VH when VH is above approximately 1.5V.

If Q1 or Q2 is externally pulled above VH, an internal

diode conducts, pulling VH a diode drop below the

output and powering anything connected to VH. This

voltage will also power the other comparator outputs.

LEVEL

TRANSLATOR

PWM

VL

BST_

DH_

LX_

DL_

VL

BATTERY

INPUT

VL

Figure 4. Boost Supply for Gate Drivers