Lo w -Vo lt a g e , P re c is io n S t e p -Do w n
Co n t ro lle r fo r P o rt a b le CP U P o w e r
MAX136
INTEGRATOR
DEFEATED
CC = REF
= 3.3V
INTEGRATOR
ACTIVE
CC = 470pF
= 3.3V
V
OUT
50
V
50
OUT
V
OUT
V
OUT
(mV)
(mV)
-50
4
-50
4
I
(A)
I
(A)
OUT
OUT
2
0
2
0
(100µs/div)
(100µs/div)
Figure 5a. Load-Transient Response with Integrator Active
Figure 5b. Load-Transient Response with Integrator Defeated
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).
Ad ju s t a b le -Ou t p u t Fe e d b a c k
(Du a l-Mo d e FB)
A fixe d , p re s e t outp ut volta g e of 2.5V a nd 3.3V is
selected when FB is connected to V
or ground. In
CC
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
= V
(1 + R1 / R2)
OUT
REF
PWM mode (SKIP = high) forces two changes on the
PWM controller. First, it disables the minimum-current
c omp a ra tor, e ns uring fixe d -fre q ue nc y op e ra tion.
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.
where V
= 1.1V nominal. Recommended normal val-
REF
ues for R2 range from 5kΩ 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.
Lo w -No is e Op e ra t io n (P WM Mo d e )
PWM mod e (SKIP = hig h) minimize s RF a nd a udio
interference in noise-sensitive applications such as hi-fi
multime d ia -e q uip p e d s ys te ms , c e llula r p hone s , 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.
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.
______________________________________________________________________________________ 17
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
