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LTC1436A

High Efficiency Low Noise Synchronous Step-Down Switching Regulators

Linear Technology 

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OPERATIO (Refer to Functional Diagram)

Power-On Reset

The POR pin is an open drain output which pulls low when

the main regulator output voltage is out of regulation.

When the output voltage rises to within 7.5% of regula-

tion, a timer is started which releases POR after 216

(65536) oscillator cycles. In shutdown, the POR output is

pulled low.

Auxiliary Linear Regulator

The auxiliary linear regulator in the LTC1436A/LTC1437A

controls an external PNP transistor for operation up to

500mA. An internal AUXFB resistive divider set for 12V

operation is invoked when AUXDR pin is above 9.5V to

allow 12V VPP supplies to be easily implemented. When

AUXDR is below 8.5V an external feedback divider may be

used to set other output voltages. Taking the AUXON pin

low shuts down the auxiliary regulator providing a conve-

nient logic controlled power supply.

LTC1436A

LTC1436A-PLL/LTC1437A

The AUX block can be used as a comparator having its

inverting input tied to the internal 1.19V reference. The

AUXDR pin is used as the output and requires an external

pull-up to a supply less than 8.5V in order to inhibit the

invoking of the internal resistive divider.

INTVCC/DRVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

of the other LTC1436A/LTC1437A circuitry is derived from

the INTVCC pin. The bottom MOSFET driver supply DRVCC

pin is internally connected to INTVCC in the LTC1436A and

externally connected to INTVCC in the LTC1437A. When

the EXTVCC pin is left open, an internal 5V low dropout

regulator supplies INTVCC power. If EXTVCC is taken above

4.8V, the 5V regulator is turned off and an internal switch

is turned on to connect EXTVCC to INTVCC. This allows the

INTVCC power to be derived from a high efficiency external

source such as the output of the regulator itself or a

secondary winding, as described in the Applications Infor-

mation section.

APPLICATIONS INFORMATION

The basic LTC1436A application circuit is shown in Figure

1, High Efficiency Step-Down Converter. External compo-

nent selection is driven by the load requirement, and

begins with the selection of RSENSE. Once RSENSE is

known, COSC and L can be chosen. Next, the power

MOSFETs and D1 are selected. Finally, CIN and COUT are

selected. The circuit shown in Figure 1 can be configured

for operation up to an input voltage of 28V (limited by the

external MOSFETs).

RSENSE Selection For Output Current

RSENSE is chosen based on the required output current.

The LTC1436A/LTC1437A current comparator has a maxi-

mum threshold of 150mV/RSENSE and an input common

mode range of SGND to INTVCC. The current comparator

threshold sets the peak of the inductor current, yielding a

maximum average output current IMAX equal to the peak

value less half the peak-to-peak ripple current ∆IL.

Allowing a margin for variations in the LTC1436A/

LTC1437A and external component values yields:

RSENSE

=

100mV

IMAX

The LTC1436A/LTC1437A work well with RSENSE values

≥ 0.005Ω.

COSC Selection for Operating Frequency

The LTC1436A/LTC1437A use a constant frequency

architecture with the frequency determined by an external

oscillator capacitor COSC. Each time the topside MOSFET

turns on, the voltage on COSC is reset to ground. During the

on-time, COSC is charged by a fixed current plus an

additional current which is proportional to the output

voltage of the phase detector VPLLLPF (LTC1436A-PLL/

LTC1437A). When the voltage on the capacitor reaches

1.19V, COSC is reset to ground. The process then repeats.

The value of COSC is calculated from the desired operating

frequency. Assuming the phase-locked loop has no exter-

nal oscillator input (VPLLLPF = 0V):

11

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