AME
3A, 28V, 340KHz Synchronous
Rectified Step-Down Converter
AME5268
Over-voltage Protection
n Detailed Description
The AME5268 has an over-voltage protection (OVP) cir-
cuit to minimize voltage overshoot when recovering from
output fault conditions. The OVP circuit include an over-
voltage comparator to compare the FB pin voltage and a
threshold of 120% x VFB. Once the FB pin voltage is higher
than the threshold, the COMP pin and the SS pin are dis-
charged to GND, forcing the high-side MOSFET off. When
the FB pin voltage drops lower than the threshold, the high-
side MOSFET will be enabled again.
Oscillator Frequency
The internal free running oscillator sets the PWM fre-
quency at 340KHz.
Enable and Soft start
The EN Pin provides electrical on/off control of the regu-
lator. Once the EN pin voltage exceeds the lockout thresh-
old voltage, the regulator starts operation and the soft start
begins to ramp. If the EN pin voltage is pulled below the
lockout threshold voltage, the regulator stops switching
and the soft start resets. Connecting the pin to ground or
to any voltage less than 0.5V will disable the regulator and
activate the shutdown mode. To limit the start-up inrush
current, a soft-start circuit is used to ramp up the refer-
ence voltage from 0V to its final value, linearly. The soft-
start time is 15 ms typically.
Thermal Shutdown
The AME5268 protects itself from overheating with an
internal thermal shutdown circuit. If the junction tempera-
ture exceeds the thermal shutdown trip point, the voltage
reference is grounded and the high-side MOSFET is turned
off. The part is restarted under control of the soft start
circuit automatically when the junction temperature drops
30OC below the thermal shutdown trip point.
Under Voltage Lockout (UVLO)
Component Selection
The AME5268 incorporates an under voltage lockout cir-
cuit to keep the device disabled when VIN (the input volt-
age) is below the UVLO start threshold voltage. During
power up, internal circuits are held inactive and the soft
start is grounded until VIN exceeds the UVLO start thresh-
old voltage. Once the UVLO start threshold voltage is
reached, the soft start is released and device start-up be-
gins. The device operates until VIN falls below the UVLO
Setting the Output Voltage
The output voltage is using a resistive voltage divider con-
nected from the output voltage to FB. It divides the output
voltage down to the feedback voltage by the ratio:
C
stop threshold voltage. The typical hysteresis in the UVLO i e
comparator is 210mV.
R
2
V
FB =VOUT
R
1
+ R
2
Over-Current Protection
the output voltage is:
Overcurrent limiting is implemented by monitoring the
current through the high side MOSFET. If this current ex-
ceeds the over-current threshold limit, the overcurrent in-
dicator is set true. The system will ignore the over-current
indicator for the leading edge blanking time at the begin-
ning of each cycle to avoid any turn-on noise glitches.
R
1
+ R
2
V
OUT = 0.925´
R
2
Once overcurrent indicator is set true. The high-side
MOSFET is turned off for the rest of the cycle after a propa-
gation delay. This over-current limiting mode is called
cycle-by-cycle current limiting.
Rev.B.01
11
AME [ ANALOG MICROELECTRONICS ]
