Rainbow Electronics MAX1672 - Manual (Page 7)

MAX1672
Step-Up/Down DC-DC Converter
in QSOP Package
_______________________________________________________________________________________ 7
_______________Detailed Description
The MAX1672 integrates a step-up, switch-mode DC-
DC converter with a linear regulator to provide step-
up/down voltage conversion. The step-up converter
contains an N-channel power MOSFET switch, while
the linear regulator contains a P-channel MOSFET pass
element (Figure 1). The step-up converter and the lin-
ear regulator share the same precision voltage refer-
ence. The MAX1672’s input range is from +1.8V to
+11V, and the regulated output is internally preset to
+3.3V or +5V, or can be adjusted with two external
resistors. Boost efficiency typically exceeds 80% over a
2mA to 200mA load range. The device is bootstrapped
with chip power derived from the stepped-up voltage
output at PS. The MAX1672 typically starts up with a
0.9V input.
The MAX1672’s step-up/linear-regulator configuration
permits the use of a physically smaller inductor than
competing SEPIC and flyback configurations because
the
1 /
2
LI 2 requirements of a step-up converter are half
those of SEPIC and flyback converters. Also, high-fre-
quency switching and selectable peak inductor current
limit allow for low inductor value (10µH) and low current
saturation rating, respectively, further reducing the
inductor’s physical dimensions.
The MAX1672 maximizes efficiency in both step-up and
step-down operation. In step-up mode, when V
IN <
V OUT , only the step-up regulator is active, while the lin-
ear regulator behaves as a 1.2
Ω
(at 5V output) PFET
switch. This provides optimum efficiency (typically
85%).
In low-dropout, step-down operation, when V IN is slight-
ly greater than V OUT , both the step-up regulator and
linear regulator are active. The step-up regulator is
automatically enabled to maintain headroom across the
linear regulator (typically 1V above the 5V output). In
this case, boost ripple is rejected by the linear regula-
tor, and OUT remains in regulation with no dropout.
In normal step-down operation, when V IN is significantly
greater than V OUT , only the linear regulator is active.
The mode of operation is automatically controlled on-
chip through the IN pin, which compares V
IN and
V OUT . Transitions between step-up, low-dropout step-
down, and normal step-down operation are stable, but
can be seen as small variations in the output DC level
and output ripple.
Step-Up Switch-Mode Converter
A pulse-frequency-modulation (PFM) control scheme,
with a constant 1µs off-time and variable on-time, con-
trols the N-channel MOSFET switch. A pulse is initiated
whenever OUT falls out of regulation. The N-channel
switch then turns off when the inductor current reaches
the peak current limit or after the 4µs maximum on-time,
whichever occurs first. This control architecture pro-
vides high-efficiency, discontinuous inductor current
under light loads as well as continuous inductor current
under heavy loads. The switching frequency and output
ripple are a function of load current and input voltage.
Linear Regulator
The low-dropout linear regulator consists of a refer-
ence, an error amplifier, and a P-channel MOSFET. The
reference is connected to the error amplifier input. The
error amplifier compares this reference with the select-
ed feedback voltage and amplifies the difference. The
difference is conditioned and applied to the P-channel
pass transistor’s gate.
ILIM
The current-limit-select input, ILIM, selects between the
two peak inductor current limits: 0.8A (ILIM = PS) and
0.5A (ILIM = GND). If the application requires low out-
put current (see
Typical Operating Characteristics ),
select 0.5A. The lower peak current limit allows for a
smaller, lower-cost inductor, and reduced output ripple.
On/Off Control
The MAX1672 is turned on or off by logic inputs ONA
and ONB (Table 1). When ONA = 1 or ONB = 0, the
device is on. When ONA = 0 and ONB = 1, the device
shuts down (see the
Applications Information section).
For normal (on) operation, connect ONA to PS and
ONB to GND. Shutdown mode turns off the MAX1672
completely, disconnecting the input from the output
and actively pulling OUT to GND.
ONB
MAX1672
0 0 On
0 1 Off
ONA
1 0 On
1 1 On
Table 1. On/Off Logic Control