Detailed Description

The MAX5096/MAX5097 are easy-to-use, high-efficien-

cy, PWM current-mode, step-down switching convert-

ers in normal operation. The MAX5096/MAX5097 have

an internal high-side p-channel 0.9Ω switch and use a

low forward-drop freewheeling diode for rectification. In

Buck Mode, the p-channel switches at the 135kHz or

330kHz frequency. Buck Mode uses a current-mode

control architecture that offers excellent line-transient

response, easier frequency compensation, and cycle-

by-cycle current limiting. The buck converter is com-

pensated externally for a selected value/type of output

inductor and capacitor.

The internal p-channel switch acts as a pass element

when operating in the low-quiescent-current LDO

Mode.

The LDO Mode can be selected on the fly through the

LDO/BUCK input. During the key-off condition, the sys-

tem’s microcontroller drives the LDO/BUCK input high

and forces the MAX5096/MAX5097 into LDO Mode,

reducing the quiescent current to 1µA (typ). When in

LDO Mode, the device is capable of delivering up to

100mA, which may be limited by the device power dis-

sipation. The LDO and switcher share the same pass

element and the reference; however, the error ampli-

fiers are different with their own compensation

schemes.

The MAX5096/MAX5097 include an integrated micro-

processor reset circuit with an adjustable reset timeout

period. The internal reset circuit monitors the regulator

output voltage and asserts RESET low when the regula-

tor output falls below the reset threshold voltage. Other

features include an enable input, externally program-

mable soft-start, optimized current-limit protection in

both LDO and Buck Modes, and thermal shutdown.

Enable Input (EN)

EN is a logic-level enable input that turns the device on

or off. The logic-high and logic-low voltages for the EN

input are 1.4V and 0.4V, respectively. Drive EN high to

turn on the device, and drive it low to place the device

in shutdown. Leaving EN unconnected disables the

device since the EN is internally pulled low with a 0.5µA

current, however, a forced pulldown of EN improves the

noise immunity. The MAX5096/MAX5097 draw 6µA

(typ) of supply current when in shutdown. EN with-

stands up to +40V, allowing EN to be connected direct-

ly to IN for always-on operation. The converter may be

turned on and off while in both Buck and LDO Modes.

Each time the EN is toggled, the output rises with a pro-

grammed soft-start period.

Internal Regulator (BP)/

Undervoltage Lockout

The MAX5096/MAX5097 include an internal 4V auxiliary

regulator to power internal circuitry. Bypass the auxil-

iary regulator output (BP) to SGND with a 1µF ceramic

capacitor physically located close to the device. The

regulator is not intended to supply the external circuit

other than pulling up the LDO/BUCK input or RESET.

Do not load BP externally by more than 2mA. The regu-

lator output is regulated to 4V with 7% accuracy during

steady state. During turn-on, the BP voltage stabilizes

after 250µs with a 1µF capacitor at BP. Drive EN high to

turn on the internal regulator. The internal UVLO with

hysteresis ensures stable operation, resulting in the

monotonic rise of the output voltage. The UVLO circuit

monitors the output of the regulator. The rising UVLO

threshold is internally set to 3.65V (BP rising) with a

185mV hysteresis (BP falling). The 3.65V UVLO at the

than 4V.

Soft-Start (SS)

Soft-start provides for the monotonic, glitch-free turn-on

of the converter. Soft-start limits the input inrush current

which may cause a glitch, especially if the source

impedance is high. The soft-start period required also

depends on the output capacitance and the closed-

loop bandwidth of converter. The soft-start period for

the MAX5096/MAX5097 is externally programmable

achieved by the controlled ramping up of the error

and the UVLO threshold is reached, the device enters

soft-start. During soft-start, 5µA is sourced into the

causing the reference voltage to ramp up slowly. When

making it ready for the next soft-start period.

t

V

I

C

SS

SS

SS

SS

=×

40V, 600mA Buck Converters with Low-

Quiescent-Current Linear Regulator Mode

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