5 V/1 2 V/1 5 V o r Ad ju s t a b le , High-Effic ie nc y,
Low I , Ste p-Up DC-DC Controlle rs
Q
_________________________________________________P in De s c rip t io n (c o n t in u e d )
PIN
MAX770
MAX771 MAX773
MAX772
NAME
FUNCTION
Low-level gate/base drive for external power transistor. Connect to the gate of an external
N-channel MOSFET or to the base of an external NPN transistor.
—
—
—
12
13
14
EXTL
EXTH
V15
High-level gate/base drive for external power transistor. Connect to EXTL when using an external
N-channel MOSFET. When using an external NPN transistor, connect a resistor R
EXTH to the base of the NPN to set the maximum base-drive current.
from
BASE
Input sense point for 15V-output operation. Connect V
to V15 for 15V-output operation.
OUT
Leave unconnected for adjustable-output operation
range is 2V to V
the external power transistor is switched from V
. The voltage applied to the gate of
OUT
_______________De t a ile d De s c rip t io n
to
0–MAX73
OUT
The MAX770–MAX773 are BiCMOS, step-up, switch-
mode power-supply controllers that provide preset 5V,
12V, and 15V output voltages, in addition to adjustable-
output operation. Their unique control scheme com-
bines the advantages of pulse-frequency modulation
(low supply current) and pulse-width modulation (high
efficiency with heavy loads), providing high efficiency
over a wide output current range, as well as increased
output current capability over previous PFM devices.
In a d d ition, the e xte rna l s e ns e re s is tor a nd p owe r
transistor allow the user to tailor the output current
capability for each application. Figure 1 shows the
MAX770–MAX773 block diagram.
ground, providing more switch gate drive and thus
reducing the transistor’s on resistance.
In non-bootstrapped mode, the IC is powered from the
input voltage (V+) and operates with minimum supply
current. In this mode, FB is the output voltage sense
point. Since the voltage swing applied to the gate of the
external power transistor is reduced (the gate swings
from V+ to ground), the power transistor’s on resistance
increases at low input voltages. However, the supply
current is also reduced because V+ is at a lower volt-
a g e , a nd b e c a us e le s s e ne rg y is c ons ume d while
charging and discharging the external MOSFET’s gate
c a p a c ita nc e . The minimum inp ut volta g e for the
MAX770–MAX773 is 3V when using external feedback
resistors. With supply voltages below 5V, bootstrapped
mode is recommended.
The MAX770–MAX773 offer three main improvements
over prior pulse-skipping control solutions: 1) the con-
verters operate with tiny (5mm height and less than
9mm diameter) surface-mount inductors due to their
300kHz switching frequency; 2) the current-limited PFM
control scheme allows 87% efficiencies over a wide
range of load currents; and 3) the maximum supply
current is only 110µA.
Note: When using the MAX770/MAX771/MAX772 in
non-bootstrapped mode, there is no preset output
operation because V+ is also the output voltage
sense point for fixed-output operation. External
resistors must be used to set the output voltage.
Use 1% external feedback resistors when operating
in adjustable-output mode (Figures 2c, 2d, 3b, 3d, 3e)
to achieve an overall output voltage accuracy of ±5%.
The MAX773 c a n b e op e ra te d in non-b oots tra p p e d
mod e without us ing e xte rna l fe e d b a c k re s is tors
because V+ does not act as the output voltage sense
point with preset-output operation. To achieve high-
est efficiency, operate in bootstrapped mode when-
ever possible.
The MAX773 can be configured to operate from an
internal 6V shunt regulator, allowing very high input/out-
p ut volta g e s . Its outp ut c a n b e c onfig ure d for a n
adjustable voltage or for one of three fixed voltages
(5V, 12V, or 15V), and it has a power-fail comparator for
low-battery detection.
All devices have shutdown capability, reducing the
supply current to 5µA max.
Bo o t s t ra p p e d /No n -Bo o t s t ra p p e d Mo d e s
Figures 2 and 3 show standard application circuits for
bootstrapped and non-bootstrapped modes. In boot-
strapped mode, the IC is powered from the output
MAX7 7 3 S h u n t -Re g u la t o r Op e ra t io n
The MAX773 has an internal 6V shunt regulator that
a llows the d e vic e to s te p up from ve ry hig h inp ut
voltages (Figure 4).
(V , which is connected to V+) and the input voltage
OUT
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8
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
