MAX770–MAX773
5V/12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controllers
8
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_______________Detailed Description
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 addition, the external sense resistor and power
transistor allow the user to tailor the output current
capability for each application. Figure 1 shows the
MAX770–MAX773 block diagram.
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.
The MAX773 can be configured to operate from an
internal 6V shunt regulator, allowing very high input/out-
put voltages. Its output can be configured for an
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.
Bootstrapped/Non-Bootstrapped Modes
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
(V
OUT
, which is connected to V+) and the input voltage
range is 2V to V
OUT
. The voltage applied to the gate of
the external power transistor is switched from V
OUT
to
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-
age, and because less energy is consumed while
charging and discharging the external MOSFET’s gate
capacitance. The minimum input voltage for the
MAX770–MAX773 is 3V when using external feedback
resistors. With supply voltages below 5V, bootstrapped
mode is recommended.
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 can be operated in non-bootstrapped
mode without using external feedback resistors
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.
MAX773 Shunt-Regulator Operation
The MAX773 has an internal 6V shunt regulator that
allows the device to step up from very high input
voltages (Figure 4).
PIN
NAME FUNCTION
MAX770
MAX771
MAX772
MAX773
— 13 EXTH
— 12 EXTL
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.
— 14 V15
Input sense point for 15V-output operation. Connect V
OUT
to V15 for 15V-output operation.
Leave unconnected for adjustable-output operation
_________________________________________________Pin Description (continued)
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
BASE
from
EXTH to the base of the NPN to set the maximum base-drive current.
