Hig h -Effic ie n c y, P WM, S t e p -Do w n
DC-DC Co n t ro lle rs in 1 6 -P in QS OP
2–MAX165
put. Adjusting the main output voltage with external
Table 3. Operating-Mode Truth Table
resistors is easy for any of the devices in this family, via
the circuit of Figure 6. The feedback voltage is nominal-
ly 2.5 for all family members except the MAX1655,
which has a nominal FB voltage of 1V. The output volt-
age (given by the formula in Figure 6) should be set
approximately 2% high in order to make up for the
MAX1652’s loa d -re g ula tion e rror. For e xa mp le , if
designing for a 3.0V output, use a resistor ratio that
results in a nominal output voltage of 3.06V. This slight
offs e tting g ive s the b e s t p os s ib le a c c ura c y.
Recommended normal values for R5 range from 5kΩ to
100kΩ.
LOAD
CURRENT
MODE
NAME
SHDN SKIP
DESCRIPTION
All circuit blocks
Low
X
X
Shutdown turned off; supply
current = 3µA typ
Pulse-skipping;
supply current =
Low,
<10%
High
Low
Idle
300µA typ at V
=
IN
10V; discontinuous
inductor current
Pulse-skipping;
continuous inductor
current
Medium,
<30%
High
High
Low
Low
Idle
Remote sensing of the output voltage, while not possi-
ble in fixed-output mode due to the combined nature of
the voltage- and current-sense input (CSL), is easy to
achieve in adjustable mode by using the top of the
external resistor divider as the remote sense point.
Constant-frequency
PWM; continuous
inductor current
High,
>30%
PWM
Constant-frequency
PWM regardless of
load; continuous
inductor current
even at no load
Du t y-Fa c t o r Lim it a t io n s fo r
Low Noise*
(PWM)
Lo w V /V Ra t io s
High
High
X
OUT IN
The MAX1652/MAX1653/MAX1654’s output voltage is
adjustable down to 2.5V and the MAX1655’s output is
adjustable as low as 1V. However, the minimum duty
factor may limit the choice of operating frequency, high
input voltage, and low output voltage.
* MAX1652/MAX1654 have no SKIP pin and therefore can’t go
into low-noise mode.
X = Don’t care
flow, a nd the re b y s up p re s s e s d is c ontinuous -mod e
inductor ringing by changing the reverse current-limit
detection threshold from 0 to -100mV, allowing the
inductor current to reverse at very light loads.
V+
REMOTE
SENSE
In most applications, SKIP should be tied to GND in
order to minimize quiescent supply current. Supply cur-
rent with SKIP high is typically 10mA to 20mA, depend-
ing on e xte rna l MOSFET g a te c a p a c ita nc e a nd
switching losses.
DH
MAIN
OUTPUT
LINES
MAX1652
MAX1653
MAX1654
MAX1655
DL
Forced continuous conduction via SKIP can improve
cross regulation of transformer-coupled multiple-output
supplies. This second function of the SKIP pin produces
a result that is similar to the method of adding sec-
ondary regulation via the SECFB feedback pin, but with
much higher quiescent supply current. Still, improving
cross regulation by enabling SKIP instead of building in
SECFB feedback can be useful in noise-sensitive appli-
cations, since SECFB and SKIP are mutually exclusive
pins/functions in the MAX1652 family.
R4
CSH
CSL
FB
GND
R5
R4
V
= V (1 + –––)
OUT
REF
R5
WHERE V (NOMINAL) = 2.5V (MAX1652–MAX1654)
REF
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 P in )
= 1.0V (MAX1655)
The MAX1652–MAX1655 fa mily ha s b oth fixe d a nd
adjustable output voltage modes. For fixed mode, con-
Figure 6. Adjusting the Main Output Voltage
nect FB to GND for a 3.3V output and to V for a 5V out-
L
______________________________________________________________________________________ 17
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
