MAX767 Maxim, MAX767 Datasheet - Page 15

• Manufacturer Part Number: MAX767
• Description: 5V-to-3.3V / Synchronous / Step-Down Power-Supply Controller
• Manufacturer: Maxim

where I
I
Note that losses in the output filter capacitors are small
when the circuit is heavily loaded, because the current
into the capacitor is not chopped. The output capacitor
sees only the small AC sawtooth ripple current. Ensure
that the input bypass capacitor has a ripple current rat-
ing that exceeds the value of I
PD
5V times the quiescent supply current (a data sheet
parameter), or about 5mW.
Under light loads, the PWM will operate in discontinu-
ous-conduction mode, where the inductor current dis-
charges to zero at some point during each switching
cycle. New loss mechanisms, insignificant at heavy
loads, begin to become important. The basic difference
is that in discontinuous mode, the AC component of the
inductor current is large compared to the load current.
This increases losses in the core and in the output filter
capacitors. Ferrite cores are recommended over pow-
dered-material types for best light-load efficiency.
At light loads, the inductor delivers triangular current
pulses rather than the nearly square waves found in
continuous-conduction mode. These pulses ramp up to
a point set by the idle-mode current comparator, which
is internally fixed at approximately 25% of the full-scale
current-limit level. This 25% threshold provides an opti-
mum balance between low-current efficiency and out-
put voltage noise (the efficiency curve would actually
look better with this threshold set at about 45%, but the
output noise would be too high).
The standard application circuit’s accuracy is dominat-
ed by reference voltage error (±1.8%) and load regula-
tion error (-2.5%). Both of these parameters can be
improved as shown in Figures 5 and 6. Both circuits
rely on an external integrator amplifier to increase the
DC loop gain in order to reduce the load regulation
error to 0.1%. Reference error is improved in the first
circuit by employing a version of the MAX767 (“T”
grade) which has a ±1.2% reference voltage tolerance.
____Additional Application Circuits
LOAD
IC
is the quiescent power dissipation of the IC and is
PD
/ 2.
RMS
CAP
= RMS AC input current, approximately
= capacitor ESR loss = I
High-Accuracy Power Supplies
______________________________________________________________________________________
5V-to-3.3V, Synchronous, Step-Down
IC Supply-Current Losses
Light-Load Efficiency
RMS
Capacitor ESR Losses
.
RMS
2
x ESR
Power-Supply Controller
Reference error of the second circuit is further
improved by substituting a highly accurate external ref-
erence chip (MAX872), which contributes ±0.38% total
error over temperature.
These two circuits were designed with the latest gener-
ation of dynamic-clock µPs in mind, which place great
demands on the transient-response performance of the
power supply. As the µP clock starts and stops, the
load current can change by several amps in less than
100ns. This tremendous ∆i/∆t can cause output voltage
overshoot or sag that results in the CPU VCC going out
of tolerance unless the power supply is carefully
designed and located close to the CPU. These circuits
have excellent dynamic response and low ripple, with
transient excursions of less than 40mV under zero to
full-load step change. In particular, these two circuits
support the “VR” (voltage regulator) version of the Intel
P54C Pentium™ CPU, which requires that its supply
voltage, including noise and transient errors, be within
the 3.30V to 3.45V range.
To configure these circuits for a given load current
requirement, substitute standard components from
Table 1 for the power switching elements (N1, N2, L1,
C1, C2) or use the Design Procedure . R1 can also be
taken from Table 1, but must be adjusted approximate-
ly 10% higher in order to maintain the correct current-
limit threshold. This increased value is due to the 0.9
gain factor introduced by the H-bridge resistor divider
(R3–R6).
If the remote sense line must sense the output voltage
on the far side of a connector or jumper that has the
possibility of becoming disconnected while the power
supply is operating, an additional 10kΩ resistor should
connect the sense line to the output voltage in the con-
nector’s power-supply side in order to prevent acciden-
tal overvoltage at the CPU.
For applications that are powered from a fixed +12V or
battery input rather than from +5V, use a MAX797 IC
instead of the MAX767. The MAX797 is capable of
accepting inputs up to 30V. See the MAX796–MAX799
data sheet for a high-accuracy circuit schematic.
15