SC4524 Datasheet, PDF (9/21 Pages) Semtech Corporation – Programmable Frequency, 2A Output 30V Step-Down Switching Regulator

SC4524
POWER MANAGEMENT
Applications Information
Setting the Output Voltage
The regulator output voltage is set with an external
resistive divider (Figure 4) with its center tap tied to the
FB pin.
The switching frequency is limited by the minimum
controllable on time at low duty cycles. For V_IN > 20V,
setting switching frequency below 500kHz makes
converter output short circuit operation more robust.
These will be described in more details later.
WPVU
Minimum On Time Consideration
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Figure 4. V_OUT is set with a Resistive Divider
The operating duty cycle of a non-synchronous step-down
switching regulator in continuous-conduction mode (CCM)
is given by
' = ^{9287 + 9'}
9_,1 + 9_' - 9_&(6$7
(2)
5_ = 5_9₂₈₇ - 
(1) ^{where VCESAT is the switch saturation voltage and VD is}
voltage drop across the rectifying diode.
The percentage error due the input bias current of the
error amplifier is
D9₂₈₇
9₂₈₇
=
- Q$ ¼ ¼ 5_ÔÏ5_
9
.
Example: Determine the output voltage error of a
9₂₈₇ = 9 converter with 5_ = NW .
From (1),
5_ = NW ¼  -  = NW
D9_{287 =} - Q$ ¼ ¼ NÔÏN _{= -}
9₂₈₇
9
.
This error is at least an order of magnitude lower than
the ratio tolerance resulting from the use of 1% resistors
in the divider string.
Setting the Switching Frequency
9
Duty cycle decreases with increasing
,1
9
ratio. In peak
287
current-mode control, the PWM modulating ramp is the
sensed current ramp of the power switch. This current
ramp is absent unless the switch is turned on. The
intersection of this ramp with the output of the voltage
feedback error amplifier determines the switch pulse
width. The propagation delay time required to
immediately turn off the switch after it is turned on is
the minimum controllable switch on time (T_ON(MIN)). Closed-
9
loop measurement of the SC4524 with low
287
9
ratios
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The switching frequency of the SC4524 is set with an
external resistor from the ROSC pin to ground. A graph
of switching frequency against R_OSC is shown in “Typical
Performance Characteristics”. The switching frequency
is programmable up to 1.5MHz.
ã 2006 Semtech Corp.
9








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Figure 5. Variation of Minimum On Time with
Ambient Temperature.
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