MAX1652 Datasheet, PDF (17/28 Pages) Maxim Integrated Products – High-Efficiency, PWM, Step-Down DC-DC Controllers in 16-Pin QSOP

High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
Table 3. Operating-Mode Truth Table
SHDN
Low
SKIP
X
LOAD
CURRENT
X
MODE
NAME
Shutdown
DESCRIPTION
All circuit blocks
turned off; supply
current = 3µA typ
High Low
Low,
<10%
Pulse-skipping;
supply current =
Idle 300µA typ at V_IN =
10V; discontinuous
inductor current
High
Low
Medium,
<30%
Pulse-skipping;
Idle continuous inductor
current
High Low
High,
>30%
PWM
Constant-frequency
PWM; continuous
inductor current
High High
Constant-frequency
X
Low Noise*
(PWM)
PWM regardless of
load; continuous
inductor current
even at no load
* MAX1652/MAX1654 have no SKIP pin and therefore can’t go
into low-noise mode.
X = Don’t care
flow, and thereby suppresses discontinuous-mode
inductor ringing by changing the reverse current-limit
detection threshold from 0 to -100mV, allowing the
inductor current to reverse at very light loads.
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 external MOSFET gate capacitance and
switching losses.
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.
Adjustable-Output Feedback
(Dual-Mode FB Pin)
The MAX1652–MAX1655 family has both fixed and
adjustable output voltage modes. For fixed mode, con-
nect FB to GND for a 3.3V output and to V_L for a 5V out-
put. Adjusting the main output voltage with external
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 load-regulation error. For example, if
designing for a 3.0V output, use a resistor ratio that
results in a nominal output voltage of 3.06V. This slight
offsetting gives the best possible accuracy.
Recommended normal values for R5 range from 5kΩ to
100kΩ.
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.
Duty-Factor Limitations for
Low V_OUT/V_IN Ratios
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.
V+
DH
MAX1652
MAX1653
MAX1654
DL
MAX1655
CSH
CSL
FB
GND
REMOTE
SENSE
LINES
MAIN
OUTPUT
R4
R5
( ) V_OUT = V_REF
1 + –R–4–
R5
WHERE V_REF (NOMINAL) = 2.5V (MAX1652–MAX1654)
= 1.0V (MAX1655)
Figure 6. Adjusting the Main Output Voltage
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