Rainbow Electronics MAX1636 - Manual (Page 17)

Adjustable-Output Feedback
(Dual-Mode FB)
A fixed, preset output voltage of 2.5V and 3.3V is
selected when FB is connected to V
CC
or ground. In
this mode, internal resistors monitor the voltage on
CSL. For voltages other than the fixed-output options,
adjust the output voltage through a resistor divider con-
nected to FB (Figure 2). Calculate the output voltage
with the following formula:
V OUT = V REF (1 + R1 / R2)
where V
REF
= 1.1V nominal. Recommended normal val-
ues for R2 range from 5k Ω to 100kΩ. To achieve a 1.1V
nominal output, simply connect FB directly to CSL.
Remote output voltage sensing is not possible in fixed
output mode due to the combined nature of the volt-
age-sense and current-sense inputs (CSL). It is, howev-
er, easy to do in adjustable mode by using the top of
the external resistor divider as the remote sense point.
Low-Noise Operation (PWM Mode)
PWM mode (SKIP = high) minimizes RF and audio
interference in noise-sensitive applications such as hi-fi
multimedia-equipped systems, cellular phones, RF
communicating computers, and electromagnetic pen-
entry systems. See the summary of operating modes in
Table 5. SKIP can be driven from an external logic signal.
PWM mode forces a constant switching frequency,
reducing interference due to switching noise by con-
centrating the emissions at a known frequency outside
the system audio or IF bands. Choose an oscillator fre-
quency for which switching frequency harmonics do
not overlap a sensitive frequency band. If necessary,
synchronize the oscillator to a tight-tolerance external
clock generator. To extend the output voltage-regula-
tion range, constant operating frequency is not main-
tained under overload or dropout conditions (see the
Overload and Dropout Operation section).
PWM mode (SKIP = high) forces two changes on the
PWM controller. First, it disables the minimum-current
comparator, ensuring fixed-frequency operation.
Second, it changes the detection threshold for reverse-
current limit from 0mV to -100mV, allowing the inductor
current to reverse at light loads. This results in fixed-fre-
quency operation and continuous inductor-current flow.
PWM mode eliminates discontinuous-mode inductor
ringing and improves cross-regulation of transformer-
coupled, multiple-output supplies.
In most applications, tie SKIP to GND to minimize qui-
escent supply current. VL supply current with SKIP high
is typically 20mA, depending on external MOSFET gate
capacitance and switching losses.
MAX1636
Low-Voltage, Precision Step-Down
Controller for Portable CPU Power
______________________________________________________________________________________ 17
0
2
4
-50
50
I OUT
(A)
V OUT
(mV)
(100 µs/div)
CC = REF
V
OUT
= 3.3V
INTEGRATOR
DEFEATED
0
2
4
-50
50
I OUT
(A)
V OUT
(mV)
(100 µs/div)
CC = 470pF
V
OUT = 3.3V
INTEGRATOR
ACTIVE
Figure 5a. Load-Transient Response with Integrator Active Figure 5b. Load-Transient Response with Integrator Defeated
Low-Voltage, Precision Step-Down
Controller for Portable CPU Power