SC1470 Datasheet, PDF (9/25 Pages) Semtech Corporation – Synchronous Buck Pseudo Fixed Frequency Power Supply Controller

SC1470
POWER MANAGEMENT
The current limit circuitry also protects against negative
over-current (i.e. when the current is flowing from the
load to PGND through the inductor and bottom MOSFET).
In this case, when the bottom MOSFET is turned on, the
phase node, LX, will be higher than PGND initially. The
SC1470 monitors the voltage at LX, and if it is greater
than a set threshold voltage of 125mV (nom.) the
bottom MOSFET is turned off. The device then waits for
approximately 2.5µs and then DL goes high for 300ns
(typ.) once more to sense the current. This repeats until
either the over-current condition goes away or the part
latches off due to output overvoltage (see Output
Overvoltage Protection).
Power Good Output
The power good output is an open-drain output and
requires a pull-up resistor. When the output voltage is
10% above or below its set voltage, PGD gets pulled low.
It is held low until the output voltage returns to within
+/-10% of the output set voltage. PGD is also held low
during start-up and will not be allowed to transition high
until soft start is over (440 switching cycles) and the
output reaches 90% of its set voltage. There is a 5µs
delay built into the PGD circuitry to prevent false
transitions.
Output Overvoltage Protection
When the output exceeds 10% of the its set voltage the
low-side MOSFET is latched on. It stays latched on and
the controller is latched off until reset (see below). There
is a 5µs delay built into the OV protection circuit to
prevent false transitions.
Output Undervoltage Protection
When the output is 30% below its set voltage the output
is latched in a tri-stated condition. It stays latched and
the controller is latched off until reset (see below). There
is a 5µs delay built into the UV protection circuit to
prevent false transitions. Note: to reset from any fault,
VCCA or EN/PSV must be toggled.
and allows switching to occur if the device is enabled.
Switching always starts with DL to charge up the BST
capacitor. With the softstart circuit (automatically)
enabled, it will progressively limit the output current (by
limiting the current out of the ILIM pin) over a
predetermined time period of 440 switching cycles.
The ramp occurs in four steps:
1) 110 cycles at 25% ILIM with double minimum off-time
(for purposes of the on-time one-shot, there is an
internal positive offset of 120mV to VOUT during this
period to aid in startup)
2) 110 cycles at 50% ILIM with normal minimum off-time
3) 110 cycles at 75% ILIM with normal minimum off-time
4) 110 cycles at 100% ILIM with normal minimum
off-time.
At this point the output undervoltage and power good
circuitry is enabled.
There is 100mV of hysteresis built into the UVLO circuit
and when VCCA falls to 4.1V (nom.) the output drivers
are shut down and tristated.
MOSFET Gate Drivers
The DH and DL drivers are optimized for driving
moderate-sized high-side, and larger low-side power
MOSFETs. An adaptive dead-time circuit monitors the DL
output and prevents the high-side MOSFET from turning
on until DL is fully off (below ~1V). Semtech’s
SmartDriver^TM FET drive first pulls DH high with a pullup
resistance of 10Ω (typ.) until LX = 1.5V (typ.). At this
point, an additional pullup device is activated, reducing
the resistance to 2Ω (typ.). This negates the need for an
external gate or boost resistor. The adaptive dead time
circuit also monitors the phase node, LX, to determine
the state of the high side MOSFET, and prevents the low
side MOSFET from turning on until DH is fully off (LX
below ~1V). Be sure there is low resistance and low
inductance between the DH and DL outputs to the gate
of each MOSFET.
Dropout Performance
POR, UVLO and Softstart
An internal power-on reset (POR) occurs when VCCA
exceeds 3V, starting up the internal biasing. VCCA
undervoltage lockout (UVLO) circuitry inhibits the
controller until VCCA rises above 4.2V. At this time the
UVLO circuitry resets the fault latch and soft-start counter,
The output voltage adjust range for continuous-
conduction operation is limited by the fixed 550ns
(maximum) minimum off-time one-shot. For best dropout
performance, use the slowest on-time setting of 200kHz.
When working with low input voltages, the duty-factor
limit must be calculated using worst-case values for on
and off times.
 2005 Semtech Corp.
9
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