ame5268 AME, Inc., ame5268 Datasheet - Page 11

• Manufacturer Part Number: ame5268
• Description: 3a, 28v, 340khz Synchronous Rectified Step-down Converter
• Manufacturer: AME, Inc.

Rev. A.01
AME5268
quency at 340KHz.
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.
cuit to keep the device disabled when V
age) is below the UVLO start threshold voltage. During
power up, internal circuits are held inactive and the soft
start is grounded until V
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 V
stop threshold voltage. The typical hysteresis in the UVLO
comparator is 210mV.
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.
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.
n Detailed Description
The EN Pin provides electrical on/off control of the regu-
The AME5268 incorporates an under voltage lockout cir-
Enable and Soft start
Under Voltage Lockout (UVLO)
Over-Current Protection
Oscillator Frequency
The internal free running oscillator sets the PWM fre-
Overcurrent limiting is implemented by monitoring the
Once overcurrent indicator is set true. The high-side
AME
IN
exceeds the UVLO start thresh-
IN
falls below the UVLO
IN
(the input volt-
i e
C
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 V
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.
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
30
nected from the output voltage to FB. It divides the output
voltage down to the feedback voltage by the ratio:
the output voltage is:
V
V
The output voltage is using a resistive voltage divider con-
Thermal Shutdown
The AME5268 has an over-voltage protection (OVP) cir-
The AME5268 protects itself from overheating with an
Over-voltage Protection
Component Selection
Setting the Output Voltage
FB
OUT
O
Rectified Step-Down Converter
C below the thermal shutdown trip point.
3A, 28V, 340KHz Synchronous
V
. 0
OUT
925
R
1
R
2
R
R
1
2
R
2
FB
R
. Once the FB pin voltage is higher
2
11