MAX797ESE ,Step-Down Controllers with Synchronous Rectifier for CPU PowerGeneral Description ________
MAX797ESE ,Step-Down Controllers with Synchronous Rectifier for CPU PowerFeaturesThe MAX796/MAX797/MAX799 high-performance, step- ' 96% Efficiencydown DC-DC converters with ..
MAX797ESE ,Step-Down Controllers with Synchronous Rectifier for CPU PowerApplications(SECFB) SKIP 2 LX15REF BSTNotebook and Subnotebook Computers 3 14GND DL4 MAX796 13PDAs ..
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MAX797ESE+ ,Step-Down Controllers with Synchronous Rectifier for CPU Powerapplications.MAX796CPE 0°C to +70°C 16 Plastic DIPThe SKIP override input allows automatic switchov ..
MAX797ESE+T ,Step-Down Controllers with Synchronous Rectifier for CPU PowerELECTRICAL CHARACTERISTICS(V+ = 15V, GND = PGND = 0V, I = I = 0A, T = 0°C to +70°C for MAX79_C, T = ..
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MAX796CPE-MAX796CSE-MAX796ESE-MAX797CPE-MAX797CSE-MAX797EPE-MAX797ESE
Step-Down Controllers with Synchronous Rectifier for CPU Power
_______________General DescriptionThe MAX796/MAX797/MAX799 high-performance, step-
down DC-DC converters with single or dual outputs
provide main CPU power in battery-powered systems.
These buck controllers achieve 96% efficiency by using
synchronous rectification and Maxim’s proprietary Idle
Mode™ control scheme to extend battery life at full-load
(up to 10A) and no-load outputs. Excellent dynamic
response corrects output transients caused by the latest
dynamic-clock CPUs within five 300kHz clock cycles.
Unique bootstrap circuitry drives inexpensive N-channel
MOSFETs, reducing system cost and eliminating the
crowbar switching currents found in some PMOS/NMOS
switch designs.
The MAX796/MAX799are specially equipped with a sec-
ondary feedback input (SECFB) for transformer-based
dual-output applications. This secondary feedback path
improves cross-regulation of positive (MAX796) or nega-
tive (MAX799) auxiliary outputs.
The MAX797 has a logic-controlled and synchronizable
fixed-frequency pulse-width-modulating (PWM) operating
mode, which reduces noise and RF interference in sensi-
tive mobile-communications and pen-entry applications.
The SKIPoverride input allows automatic switchover to
idle-mode operation (for high-efficiency pulse skipping) at
light loads, or forces fixed-frequency mode for lowest noise
at all loads.
The MAX796/MAX797/MAX799 are all available in 16-
pin DIP and narrow SO packages. See the table below
to compare these three converters.
________________________ApplicationsNotebook and Subnotebook Computers
PDAs and Mobile Communicators
Cellular Phones
____________________________Features96% Efficiency4.5V to 30V Input Range2.5V to 6V Adjustable OutputPreset 3.3V and 5V Outputs (at up to 10A)Multiple Regulated Outputs+5V Linear-Regulator OutputPrecision 2.505V Reference OutputAutomatic Bootstrap Circuit150kHz/300kHz Fixed-Frequency PWM OperationProgrammable Soft-Start375µA Typ Quiescent Current (VIN= 12V, VOUT= 5V)1µA Typ Shutdown Current
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU PowerIdle Mode is a trademark of Maxim Integrated Products.
†U.S. and foreign patents pending.
__________________Pin Configuration
Ordering Information continued at end of data sheet.*Contact factory for dice specifications.
______________Ordering Information
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, TA= 0°C to +70°C for MAX79_C, TA= 0°C to +85°C for MAX79_E, = -55°C to +125°C for MAX79_M, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
V+ to GND.................................................................-0.3V, +36V
GND to PGND........................................................................±2V
VL to GND...................................................................-0.3V, +7V
BST to GND...............................................................-0.3V, +36V
DH to LX...........................................................-0.3V, BST + 0.3V
LX to BST.....................................................................-7V, +0.3V
SHDNto GND............................................................-0.3V, +36V
SYNC, SS, REF, FB, SECFB, SKIP, DL to GND..-0.3V, VL + 0.3V
CSH, CSL to GND.......................................................-0.3V, +7V
VL Short Circuit to GND..............................................Momentary
REF Short Circuit to GND...........................................Continuous
VL Output Current...............................................................50mA
Continuous Power Dissipation (TA= +70°C)
SO (derate 8.70mW/°C above +70°C)........................696mW
Plastic DIP (derate 10.53mW/°C above +70°C).........842mW
CERDIP (derate 10.00mW/°C above +70°C)..............800mW
Operating Temperature Ranges
MAX79_C_ _......................................................0°C to +70°C
MAX79_E_ _....................................................-40°C to +85°C
MAX79_MJE.................................................-55°C to +125°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
Note 1:Since the reference uses VL as its supply, V+ line-regulation error is insignificant.
Note 2:At very low input voltages, quiescent supply current may increase due to excess PNP base current in the VL linear
regulator. This occurs only if V+ falls below the preset VL regulation point (5V nominal). See the Quiescent Supply Current
vs. Supply Voltage graph in the Typical Operating Characteristics.
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, TA= 0°C to +70°C for MAX79_C, TA= 0°C to +85°C for MAX79_E, = -55°C to +125°C for MAX79_M, unless otherwise noted.)
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, TA= -40°C to +85°C for MAX79_E, unless otherwise noted.) (Note 3)
Note 3:All -40°C to +85°C specifications above are guaranteed by design.
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
__________________________________________________Typical Operating Circuits
__________________________________________Typical Operating Characteristics(TA = +25°C, unless otherwise noted.)
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
_____________________________________Typical Operating Circuits (continued)
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
____________________________Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
____________________________Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
______________________________________________________________Pin DescriptionDual Mode is a trademark of Maxim Integrated Products.
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
______Standard Application CircuitIt is easy to adapt the basic MAX797 single-output 3.3V
buck converter (Figure 1) to meet a wide range of
applications with inputs up to 28V (limited by choice of
external MOSFET). Simply substitute the appropriate
components from Table 1. These circuits represent a
good set of tradeoffs between cost, size, and efficiency
while staying within the worst-case specification limits
for stress-related parameters such as capacitor ripple
current. Each of these circuits is rated for a continuous
load current at TA= +85°C, as shown. The 1A, 2A and
10A applications can withstand a continuous output
short-circuit to ground. The 3A and 5A applications can
withstand a short circuit of many seconds duration, but
the synchronous-rectifier MOSFET overheats, exceed-
ing the manufacturer’s ratings for junction temperature
by 50°C or more.
If the 3A or 5A circuit must be guaranteed to withstand
a continuous output short circuit indefinitely, see the
section MOSFET Switchesunder Selecting Other
Components. Don’t change the frequency of these cir-
cuits without first recalculating component values (par-
ticularly inductance value at maximum battery voltage).
_______________Detailed DescriptionThe MAX796 is a BiCMOS, switch-mode power-supply
controller designed primarily for buck-topology regula-
tors in battery-powered applications where high effi-
ciency and low quiescent supply current are critical.
The MAX796 also works well in other topologies such
as boost, inverting, and CLK due to the flexibility of its
floating high-speed gate driver. Light-load efficiency is
enhanced by automatic idle-mode operation—a vari-
able-frequency pulse-skipping mode that reduces
Figure 1. Standard 3.3V Application Circuit
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
Table 1. Component Selection for Standard 3.3V Applications
Table 2. Component Suppliers* Distributor
losses due to MOSFET gate charge. The step-down
power-switching circuit consists of two N-channel
MOSFETs, a rectifier, and an LC output filter. The out-
put voltage is the average of the AC voltage at the
switching node, which is adjusted and regulated by
changing the duty cycle of the MOSFET switches. The
gate-drive signal to the N-channel high-side MOSFET
must exceed the battery voltage and is provided by a
flying capacitor boost circuit that uses a 100nF capaci-
tor connected to BST.
The MAX796 contains nine major circuit blocks, which
are shown in Figure 2.
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power PWM Controller Blocks:Multi-Input PWM ComparatorCurrent-Sense CircuitPWM Logic BlockDual-Mode Internal Feedback MuxGate-Driver OutputsSecondary Feedback Comparator
Bias Generator Blocks: +5V Linear RegulatorAutomatic Bootstrap Switchover Circuit +2.505V Reference
These internal IC blocks aren’t powered directly from
the battery. Instead, a +5V linear regulator steps down
the battery voltage to supply both the IC internal rail (VL
pin) as well as the gate drivers. The synchronous-
switch gate driver is directly powered from +5V VL,
while the high-side-switch gate driver is indirectly pow-
ered from VL via an external diode-capacitor boost cir-
cuit. An automatic bootstrap circuit turns off the +5V
linear regulator and powers the IC from its output volt-
age if the output is above 4.5V.
PWM Controller BlockThe heart of the current-mode PWM controller is a
multi-input open-loop comparator that sums three sig-
nals: output voltage error signal with respect to the ref-
erence voltage, current-sense signal, and slope
compensation ramp (Figure 3). The PWM controller is a
direct summing type, lacking a traditional error amplifi-
er and the phase shift associated with it. This direct-
summing configuration approaches the ideal of
cycle-by-cycle control over the output voltage.
Under heavy loads, the controller operates in full PWM
mode. Each pulse from the oscillator sets the main
PWM latch that turns on the high-side switch for a peri-
od determined by the duty factor (approximately
VOUT/VIN). As the high-switch turns off, the synchro-
nous rectifier latch is set. 60ns later the low-side switch
turns on, and stays on until the beginning of the next
clock cycle (in continuous mode) or until the inductor
current crosses zero (in discontinuous mode). Under
fault conditions where the inductor current exceeds the
100mV current-limit threshold, the high-side latch
resets and the high-side switch turns off.
At light loads (SKIP= low), the inductor current fails to
exceed the 30mV threshold set by the minimum-current
comparator. When this occurs, the controller goes into
idle mode, skipping most of the oscillator pulses in
order to reduce the switching frequency and cut back
gate-charge losses. The oscillator is effectively gated
off at light loads because the minimum-current com-
parator immediately resets the high-side latch at the
beginning of each cycle, unless the feedback signal
falls below the reference voltage level.
When in PWM mode, the controller operates as a fixed-
frequency current-mode controller where the duty ratio
is set by the input/output voltage ratio. The current-
mode feedback system regulates the peak inductor
current as a function of the output voltage error signal.
Since the average inductor current is nearly the same
as the peak current, the circuit acts as a switch-mode
transconductance amplifier and pushes the second
output LC filter pole, normally found in a duty-factor-
controlled (voltage-mode) PWM, to a higher frequency.
To preserve inner-loop stability and eliminate regenera-
tive inductor current “staircasing,” a slope-compensa-
tion ramp is summed into the main PWM comparator to
reduce the apparent duty factor to less than 50%.
The relative gains of the voltage- and current-sense
inputs are weighted by the values of current sources
that bias three differential input stages in the main PWM
comparator (Figure 4). The relative gain of the voltage
comparator to the current comparator is internally fixed
at K = 2:1. The resulting loop gain (which is relatively
low) determines the 2.5% typical load regulation error.
The low loop-gain value helps reduce output filter
capacitor size and cost by shifting the unity-gain
crossover to a lower frequency.
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power The output filter capacitor C2 sets a dominant pole in
the feedback loop. This pole must roll off the loop gain
to unity before the zero introduced by the output
capacitor’s parasitic resistance (ESR) is encountered
(see Design Proceduresection). A 60kHz pole-zero
cancellation filter provides additional rolloff above the
unity-gain crossover. This internal 60kHz lowpass com-
pensation filter cancels the zero due to the filter capaci-
tor’s ESR. The 60kHz filter is included in the loop in
both fixed- and adjustable-output modes.
Synchronous-Rectifier Driver (DL Pin) Synchronous rectification reduces conduction losses in
the rectifier by shunting the normal Schottky diode with
a low-resistance MOSFET switch. The synchronous rec-
tifier also ensures proper start-up of the boost-gate driv-
er circuit. If you must omit the synchronous power
MOSFET for cost or other reasons, replace it with a
small-signal MOSFET such as a 2N7002.
If the circuit is operating in continuous-conduction
mode, the DL drive waveform is simply the complement
of the DH high-side drive waveform (with controlled
dead time to prevent cross-conduction or “shoot-
through”). In discontinuous (light-load) mode, the syn-
chronous switch is turned off as the inductor current
falls through zero. The synchronous rectifier works
under all operating conditions, including idle mode.
The synchronous-switch timing is further controlled by
the secondary feedback (SECFB) signal in order to
improve multiple-output cross-regulation (see
Secondary Feedback-Regulation Loopsection).
Internal VL and REF SuppliesAn internal regulator produces the 5V supply (VL) that
powers the PWM controller, logic, reference, and other
blocks within the MAX796. This +5V low-dropout linear
regulator can supply up to 5mA for external loads, with
a reserve of 20mA for gate-drive power. Bypass VL to
GND with 4.7µF. Important: VL must not be allowed to
exceed 6V. Measure VL with the main output fully
loaded. If VL is being pumped up above 5.5V, the
probable cause is either excessive boost-diode capaci-
tance or excessive ripple at V+. Use only small-signal
diodes for D2 (1N4148 preferred) and bypass V+ to
PGND with 0.1µF directly at the package pins.
The 2.505V reference (REF) is accurate to ±1.6% over
temperature, making REF useful as a precision system
reference. Bypass REF to GND with 0.33µF minimum.
REF can supply up to 1mA for external loads. However,
if tight-accuracy specs for either VOUT or REF are
essential, avoid loading REF with more than 100µA.
Loading REF reduces the main output voltage slightly,
according to the reference-voltage load regulation
error. In MAX799 applications, ensure that the SECFB
divider doesn’t load REF heavily.
When the main output voltage is above 4.5V, an internal P-
channel MOSFET switch connects CSL to VL while simul-
taneously shutting down the VL linear regulator. This
action bootstraps the IC, powering the internal circuitry
from the output voltage, rather than through a linear regu-
lator from the battery. Bootstrapping reduces power dissi-
pation caused by gate-charge and quiescent losses by
providing that power from a 90%-efficient switch-mode
source, rather than from a 50%-efficient linear regulator.
MAX796/MAX797/MAX799
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 Operationsection).
Low-Noise Mode (SKIPPin)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. SKIPcan 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-
Soundblaster is a trademark of Creative Labs.
