5 V/3 .3 V o r Ad ju s t a b le , 1 0 0 % Du t y-Cyc le ,
Hig h -Effic ie n c y, S t e p -Do w n DC-DC Co n t ro lle rs
A
B
A
B
C
C
0A
0A
10µs/div
10µs/div
CIRCUIT OF FIGURE 1, V+ = 8V, V
= 5V, LOAD = 1.5A
CIRCUIT OF FIGURE 1, V+ = 8V, V
= 5V, LOAD = 100mA
OUT
OUT
A: MOSFET DRAIN, 5V/div
A: MOSFET DRAIN, 5V/div
B: OUT, 50mV/div, 5V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
B: OUT, 50mV/div, 5V DC OFFSET
C: INDUCTOR CURRENT, 1A/div
/MAX1627
Figure 3. Discontinuous-Conduction Mode, Light-Load-Current
Waveform
Figure 4. Continuous-Conduction Mode, Heavy-Load-Current
Waveform
shutdown. Bypass the reference with 0.1µF for normal
operation. Place the bypass capacitor within 0.2 inches
(5mm) of REF, with a direct trace to GND (Figure 7).
EXT Drive Vo lt a g e Ra n g e
EXT swings from V+ to GND and provides the gate
drive for an external P-channel power MOSFET. A high-
e r s up p ly volta g e inc re a s e s the g a te d rive to the
S o ft -S t a rt
Soft-start reduces stress and transient voltage slumps
on the power source. When the output voltage is near
ground, the minimum off-time is lengthened to limit peak
switching current. This compensates for reduced nega-
tive inductor current slope due to low output voltages.
MOSFET and reduces on-resistance (R
External Switching Transistor section.
). See
DS(ON)
Qu ie s c e n t Cu rre n t
The d e vic e ’s typ ic a l q uie s c e nt c urre nt is 70µA.
However, actual applications draw additional current to
supply MOSFET switching currents, OUT pin current, or
external feedback resistors (if used), and both the diode
and capacitor leakage currents. For example, in the cir-
________________De s ig n In fo rm a t io n
S e t t in g t h e Ou t p u t Vo lt a g e
The MAX1626’s output voltage can be selected to 3.3V
or 5V under logic control by using the 3/5 pin. The 3/5
pin requires less than 0.5V to ensure a 3.3V output, or
more than (V+ - 0.5)V to guarantee a 5V output. The
voltage sense pin (OUT) must be connected to the out-
put for the MAX1626.
cuit of Figure 1, with V+ at 7V and V
at 5V, typical
OUT
no-load supply current for the entire circuit is 84µA.
When designing a circuit for high-temperature opera-
tion, select a Schottky diode with low reverse leakage.
S h u t d o w n Mo d e
When SHDN is high, the device enters shutdown mode.
In this mode, the feedback and control circuit, reference,
and internal biasing circuitry are turned off. EXT goes
high, turning off the external MOSFET. The shutdown
supply current drops to less than 1µA. SHDN is a logic-
level input. Connect SHDN to GND for normal operation.
The MAX1627’s output voltage is set using two resis-
tors, R2 and R3 (Figure 5), which form a voltage divider
between the output and GND. R2 is given by:
V
OUT
R2 = R3 x
−1
V
REF
Re fe re n c e
The 1.3V reference is suitable for driving external loads,
such as an analog-to-digital converter. It has a guaran-
teed 10mV maximum load regulation while sourcing load
currents up to 100µA. The reference is turned off during
where V
= 1.3V. Since the input bias current at FB
REF
has a maximum value of 50nA, large values (10kΩ to
200kΩ) can be used for R3 with no significant accuracy
loss. For 1% error, the current through R2 should be at
8
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