Rainbow Electronics MAX8775 - Manual (Page 17)

without a traditional error amplifier and the phase shift
associated with it.
The MAX8775 uses a relatively low loop gain, allowing
the use of lower cost output capacitors. The relative
gain of the voltage comparator to the current compara-
tor is internally fixed at 4:1. The high current gain
results in stable operation even with low-output ESR
capacitors. An internal integrator corrects for any load-
regulation error caused by the high current gain. The
low value of loop gain helps reduce output filter capaci-
tor size and cost by shifting the unity-gain crossover
frequency to a lower level.
Frequency Selection (FSEL)
The OSC input programs the PWM mode switching fre-
quency. Connect a resistor (R
OSC ) between OSC and
AGND to set the switching frequency (per phase):
f SW = 300kHz x 143kΩ / R OSC
R OSC values between 71.5kΩ and 432kΩ correspond to
switching frequencies of 600kHz to 100kHz, respectively.
High-frequency (600kHz) operation optimizes the appli-
cation for the smallest component size, trading off effi-
ciency due to higher switching losses. This may be
acceptable in ultra-portable devices where the load
currents are lower. Low-frequency (100kHz) operation
offers the best overall efficiency at the expense of com-
ponent size and board space.
MAX8775
Dual and Combinable Graphics Core
Controller for Notebook Computers
______________________________________________________________________________________ 17
REF
REFIN1
PGOOD1
ON1 = ON2
REFIN2
V OUT2
V OUT1
PGOOD2 20 μs
20 μs
TRACKING STARTUP/SHUTDOWN TIMING
REF
REFIN1
REFIN2
AGND
SLEW1
SLEW2 C SLEW1 = C SLEW2
1/5 PROGRAMMED
SLEW RATE
1/5 PROGRAMMED
SLEW RATE
MAX8775
C REF
Figure 5. MAX8775 Tracking Startup/Shutdown Timing
REF
REFIN1
PGOOD1
ON1 = ON2
REFIN2
V
OUT2
V OUT1
PGOOD2 20 μs
RATIOMETRIC STARTUP/SHUTDOWN TIMING
REF
REFIN1
REFIN2
AGND
SLEW1
SLEW2 C SLEW1 > C SLEW2
1/5 PROGRAMMED
SLEW RATE
1/5 PROGRAMMED
SLEW RATE
MAX8775
C REF
Figure 6. MAX8775 Ratiometric Startup/Shutdown Timing