Detailed Description
The MAX5096/MAX5097 are easy-to-use, high-efficiency,
PWM current-mode, step-down switching converters
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 archi-
tecture that offers excellent line-transient response, eas-
ier frequency compensation, and cycle-by-cycle current
limiting. The buck converter is compensated 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 dissipation. The
LDO and switcher share the same pass element and the
reference; however, the error amplifiers 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 regulator
output falls below the reset threshold voltage. Other fea-
tures include an enable input, externally programmable
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 sup-
ply current when in shutdown. EN withstands up to +40V,
allowing EN to be connected directly 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 tog-
gled, the output rises with a programmed soft-start period.
Internal Regulator (BP)/Undervoltage Lockout
The MAX5096/MAX5097 include an internal 4V auxiliary
regulator to power internal circuitry. Bypass the auxiliary
regulator output (BP) to SGND with a 1μF ceramic capaci-
tor 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 regulator 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 inter-
nal 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 no-load BP output guarantees
operation at VIN lower 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 using a
single capacitor (CSS). The soft-start is achieved by the
controlled ramping up of the error amplifier reference
input. At startup, after VIN is applied and the UVLO
threshold is reached, the device enters soft-start. During
soft-start, 5μA is sourced into the capacitor (CSS) con-
nected from SS to SGND (Figure 2) causing the reference
voltage to ramp up slowly. When VSS reaches 1.237V, the
output becomes fully active. Set the soft-start time (tSS)
using following equation:
SS
SS SS
SS
tC
I
= ×
where VSS is 1.237V, ISS is 5μA, tSS is in seconds, and
CSS is in Farads.
Pulling EN low quickly discharges the CSS capacitor,
making it ready for the next soft-start period.
MAX5096/MAX5097 40V, 600mA Buck Converters with Low-
Quiescent-Current Linear Regulator Mode
www.maximintegrated.com Maxim Integrated
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