Trip le -Ou t p u t P o w e r-S u p p ly
Co n t ro lle r fo r No t e b o o k Co m p u t e rs
MAX782
Boost Gate-Driver Supply
BATTERY
Gate-drive voltage for the high-side N-channel switch is
INPUT
generated with a flying-capacitor boost circuit as shown
in Figure 4. The capacitor is alternately charged from
the VL supply via the diode and placed in parallel with
the high-side MOSFET’s gate-source terminals. On start-
up, the synchronous rectifier (low-side) MOSFET 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 connecting the capacitor to the MOSFET
gate 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.
VL
VL
BST_
DH_
LX_
LEVEL
TRANSLATOR
PWM
VL
DL_
Ringing seen at the high-side MOSFET gates (DH3 and
DH5) in discontinuous-conduction mode (light loads) is
a natural operating condition caused by the residual
energy in the tank circuit formed by the inductor and
stray capacitance at the LX_ nodes. The gate driver
negative rail is referred to LX_, so any ringing there is
directly coupled to the gate-drive supply.
Figure 4. Boost Supply for Gate Drivers
c ond ition whe ne ve r the loa d c urre nt is le s s tha n
approximately 25% of full load.
Modes of Operation
PWM Mode
Under heavy loads – over approximately 25% of full load
– the +3.3V and +5V supplies operate as continuous-cur-
re nt PWM s up p lie s (s e e Typ ic a l Op e ra ting
Characteristics). The duty cycle (%ON) is approximately:
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,
s hort b urs ts of p uls e s oc c ur tha t ma ke the c urre nt
waveform look erratic, but do not materially affect the
output ripple. Efficiency remains high.
%ON = V /V
OUT IN
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
a s e ne rg y is p ut into the ind uc tor a nd the n d is -
charged into the load. Note that the current flowing
into the ind uc tor whe n it is b e ing c ha rg e d is a ls o
flowing into the loa d , s o the loa d is c ontinuous ly
receiving current from the inductor. This minimizes
output ripple and maximizes inductor use, allowing
very small physical and electrical sizes. Output rip -
ple is primarily a function of the filter capacitor (C7 or
C6) effective series resistance (ESR) and is typically
und e r 50mV (s e e the De s ig n Proc e d ure s e c tion).
Outp ut rip p le is wors t a t lig ht loa d a nd ma ximum
input voltage.
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 induc-
tor, allowing the inductor current to be continuously mea-
sured 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 resistor R1 (R2)
is typically 25mΩ (20mΩ) for 3A load current.
Oscillator Frequency; SYNC Input
The SYNC inp ut c ontrols the os c illa tor fre q ue nc y.
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.
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
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).
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