MIC25400 Datasheet, PDF (12/27 Pages) Micrel Semiconductor – 2A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
The following steps can be taken to lower the gate drive
impedance, minimize the dv/dt-induced current and
lower the FET’s susceptibility to the induced glitch:
• Chose a low-side MOSFET with a high C_GS/C_GD
ratio and a low internal gate resistance
• Do not put a resistor between the LSD output
and the gate.
• Insure both the gate drive and return etch are
short, low inductance connections.
• Use a 4.5V V_GS rated MOSFET. Its higher gate
threshold voltage is more immune to glitches
than a 2.5V or 3.3V rated MOSFET. MOSFETs
that are rated for operation at less than 4.5 V_GS
should not be used.
• Add a resistor in series with the BST pin. This
will slow down the turn-on time of the high-side
MOSFET while leaving the turn-off time
unaffected.
Current Limit
The MIC25400 uses the synchronous (low-side)
MOSFET’s RDS_ON to sense an over-current condition.
The low-side MOSFET is used because it displays lower
parasitic oscillations after switching than the upper
MOSFET. Additionally, it improves the accuracy and
reduces false tripping at lower voltage outputs and
narrow duty cycles since the off-time increases as duty-
cycle decreases. Figure 7 shows how over current
protection is performed using the low-side MOSFET.
MIC25400
an offset voltage (I_CS x R_CS) that is compared to the V_DS
of the low-side FET. When I_SD (source-to-drain current) x
RDS_ON is equal to this voltage the soft-start circuit is
reset and a hiccup current mode is initiated to protect the
power supply and load from excessive current during
short circuits.
Current Limit Calculations and Maximum Peak Limit
The current limit method requires careful selection of the
inductor value and saturation current. If a short circuit
occurs during the off-time, the overcurrent circuit will
take up to a full cycle to detect the overcurrent once it
exceed the overcurrent limit. The worst case occurs if
the output current is 0A and a hard short is applied to the
output. The short circuit causes the output voltage to fall,
which increases the pulse width of the regulator. It may
take three or four cycles for the current to build up in the
inductor before current limit forces the part into hiccup
mode. The wider pulse width generates a larger peak to
peak inductor current which can saturate the inductor.
For this reason, the minimum inductor value for the
MIC25400 is 4.7µH and the maximum peak current limit
setpoint is 2.7A. The saturation current for each of these
inductors should be at least 1.5A higher than the
overcurrent limit setting.
Voltage Setting Components
The regulator requires two external resistors to set the
output voltage as shown in Figure 8.
Figure 7. Over-current Circuit
Inductor current, I_L, flows from the lower MOSFET
source to the drain during the off-time, causing the drain
voltage to become negative with respect to ground. This
negative voltage is proportional to the instantaneous
inductor current times the MOSFET RDS_ON. The low-
side MOSFET voltage becomes even more negative as
the output current increases.
The over-current circuit operates by passing a known
fixed current source through a resistor R_CS. This sets up
Figure 8. Setting the Output Voltage
The output voltage is determined by the equation below.
V_OUT
=
V_REF
× ⎜⎛1 + ^R1⎟⎞
⎝ ^R2 ⎠
Where: V_REF is 0.7V nominal.
If the voltage divider resistance is used to provide the
minimum load (see EN/DLY section) then R1 should be
low enough to provide the necessary impedance.
Once R1 is selected, R2 can be calculated with the
following formula:
January 2011
12
M9999-020111-C